How big is the red area? What length of needle and how thick is the fat/skin at your injection spot?
Kinda sounds like the result of sub-q oil. (I've had this happen when using 1/2" slin pins and not making sure that the needle is perpendicular enough on shoulder shots.)
I doubt if heating the oil has much to do with it but you could always check by doing everything the same except using, say, enanthate instead of prop.
It's gone now thankfully.
The areas would be about 1x1 inch at the site of injection.
IM with 25g 1.5 inch, 3ml inject at a time, which I think is seeping out of the muscle.
I think you're right, on the sub-q, that's why I'm going to start using the z-track method.