Injecting - What the heck! Burns when injecting!

[Wallet]

I got an email from a friend today. I will copy and paste it here:

I have beeb injecting heroin regularly for a couple months now. Every once in a while I will inject it too soon after cooking it and may get a bit of a stinging sensation. Nothing out of control and it is explainable since the liquid is too hot. However, I have never experienced before what I have last night and today. Last night I cooked the H and filled up the needle as usual. I found the vein and began to inject the liquid but only for a nano second. As soon as I did I felt a terrible burn in my arm. I almost screamed! I stoped and felt the liquid. It was cool enough. So I tried again a minute later and it was the exact same problem. I put the needle and spoon away and got a fresh needle and spoon. Cooked up a different batch and had no problem. Injected fine, no pain.

This morning however the same thing happened with the burning. I used the same spoon I did last night AFTER the burning incident (so I used the one that didn't hurt me). However, as soon as I started to inject the pain was awful! I again switched spoons, needles and bags of dope and the same thing happened! I had no idea what was going on. It hurt so badly I couldn't possibly inject it. So I tried again, with a different batch and somehow it worked fine this time. No pain. However, now I'm worried about the pain happening again since as of right now it seems to be randomly happening. Any idea what could cause this?

Well that's the end of the email. Anyone have any idea what's up with the dude? What can cause this burning? Would not using enough water do it? Like maybe the H isn't fully dissolved so it burns going into the blood? Any other ideas?