Thats a hard Q to A man. I will give you alist of things to check. Check them off 1 by one and you will get to the source of problem.
What type of pin are you using?
What type of concentration are you running?
How much BA & or BB you using?
How sanitary is the substance, was it filtered later thru a sterile watman?
IF so how thick is the liquid will you be able to shoot it thru 21-25 gage?
How fast were you shooting?
Did you aspirate?
Did you shoot in the correct spot?
The reason im asking you these questions is so we get to the bottom of what is causing the pain. All properly made vitamins should be injected without pain, if there is pain it is unwise to just " heat the shit up , etc" or "cool this shit down".
My 2 cents is that you probibaly got too much ba in there or its not filtered properly.
If all else fails shoot slowly an dput the pin in the fridge for a while.
Peace