"When in doubt, get it checked out"...
That being said it does not sound very serious.
I was an EMT, trained and served as a combat medic, and am very skilled in venipuncture.
A few things to look out for:
Abscess-a hottish mass beneath the surface of the skin, looks like a goose egg at the injection site, forming over a few days. This is the most serious situation, because basically what has happened is you have introduced god-knows-what bacteria under the skin where it has grown a possibly deadly amount of colonies and toxins, and can affect all sorts of crap. If left untreated, it can infect the bone. This is the one that people loose their arms over. You could also go septic and die. Method of treatment is somewhat harsh; If the Dr. is nice, he will try to numb it, but abscesses are notoriously difficult to numb. Then the abscess is lanced, or cut or sliced, then vigorously squeezed like a zit until the blood coming out no longer is discolored or stinks. follow up with oral antibiotics.
P.S. This does not happen in the space of a few hours, but rather days. Perhaps the first signs might be there at T+24hrs.
Blown vein- This is where the needle goes all the way through the vein and pops out the other side spilling blood under the skin, or the vein rips open when punctured. Because of the increased pressure due to the tourniquet and the patients B/P, the vein can rip open and spill quite a little blood in a short time until the building pressure stops it or the vein clots up. Let go of the tourniquet and apply pressure immediately to minimize symptoms. If the pooling blood is close to a nerve, this can be quite a painful situation, and look real nasty. This is, however, a very common situation, embarrassing even the most skilled surgeons and Nurses. Other than the pain, unsightly bruise, and not being able to shoot in that arm for a while, ('bout 10 min.), it's harmless,(unless you're hemophiliac).
Thrombosis- This one can be a little more dangerous, (and unbelievable painful), especially in the case of DVT's, or "Deep Vein Thrombosis". This occurs when there is a blockage in the actual vein itself, sometimes clotted blood, sometimes crap in the rig. The danger with this one is if the clot or blockage forms in specific parts of the veins, it "THEORETICALLY" can break loose and float around in your circulatory system, lodge in the tiny veins or arteries servicing the heart or brain, stopping blood flow, causing strokes, embolism, Myo-Infarct. That being said, I was assured by a quite knowledgeable MD once that this could not happen when discussing thrombosis in the superficial veins in the arm. I've not read the research, so I don't know how reliable that info was. In order to avoid this one, simply boil a couple of coffee filters in a cup in the microwave. Mix up swiy H, (or X.Y.Z.), and run it through the filters. H hydrochloride is soluble in water. Larger, non-solubles will get trapped, protecting the veins. SWIYou can also, and swim recommends, purchasing Synge filters that will even strain bacteria. Just a side note, some substances will go into solution, then crash out after filtering, causing a nasty sludge in the rig, clogging veins.
Collapsed Vein-This is exactly what it sounds like. When someone sticks a frigging dull overused needle in the same vein at the same spot four times a day for a week straight because he is too lazy to rotate his sites and he happened upon a good one, the lining of the vein can become irritated and swell blocking the flow of blood, (sort of like our thrombosis), causing everything to "collapse" and the sides of the vein to stick together downstream of the blockage. It can also happen when one draws too hard on the plunger to check placement. Usually, if the cause clears up, circulation can and usually will be restored. If not, as in the case of our thrombosis, the damaged vein will be reabsorbed, and the blood pressure from the undamaged portion of the vein will prompt a new section of the vein to grow.
Skin Popping-This is actually a legit injection technique, but can be painful and somewhat wasteful if only a small amount of medicine is used. This occurs when the needle is beneath the skin and not in the vein. Plunger pushed down, fluid collects under the skin forming a bubble. This is called a sub-cutaneous injection. Usually one would choose this route because it allows one to take a large dose without stopping the heart or respiratory system, and with opiates it extends the euphoria sometimes by hours. Also, with skin popping, you could get abscesses, but very little risk of an infection around the heart, a systemic blood infection, and makes it easier to hide ones habit than holes and bruises on the arm. It does not give the orgasmic rush that hitting a vein does, but its advantages make it worthwhile. Use the fatty tissues high on the gluts do bang even up to 10cc, (a little ouch), a spot.
There are other things that can happen when one decides to go this route, ( least of which is a very easy O.D.), but I've covered the most common ones.
Look, here is the thing. I.V. can be a wonderful route of admin for many drugs, but one must understand the dangers and the risks, take precautions, and respect what it is swiy decided to do.
Your Biggest Risk-Infection is what kills I.V. junkies and makes them loose their limbs. Use new rigs when able, clean everything with bleach-water and lots of soap. Boil I.V. dilutant. Watch out for signs of infection, (persistent redness, soreness, change/increase in size, shape, color. or temperature), in the skin surrounding an injection site.