I'm no expert, but I want to throw this out there.
I'm on my first cycle, 500/mg of Test E per week split into 2 injections. For each muscle that it was my first injection into that muscle, the "virgin muscle" effect was intense. I was nearly limping for 3 days after (not painful really, just crazy stiff). Subsequent injections were less sore, but whatever muscle got the most recent pin would always be sore, tender and tight for 2-3 days afterwards.
One day, because of my schedule, I wasn't going to be able to pin my usual hour before the gym. Instead I pinned at lunch (gym sesh was later in the evening). My usual post injection soreness (I want to point out here that I think the "pain" part of PIP is a misnomer) was much less. I chalked this up to just getting better at my injection technique and went back to my usual timing of 1 hour before the gym.
Fast forward another 2 weeks. I got sick. Nothing crazy, just a little sore throat and sniffles. I also pulled a bicep deadlifting (common, minor powerlifting injury, takes 3 days to heal tops if you take care of it). I cut that workout short, and then delayed my next training day by 1 day to get in a little extra rest for the cold and pulled bicep. I didn't see any reason why I should alter my dosing schedule though, so I pinned on a day I did not train. What happened next was magical: next to zero post injection soreness/tenderness/stiffniss. Ok, so it was still slightly noticeable, but I would say the soreness was a mere 10% of what it was when I was pinning an hour before training.
The more I think about this, the more it makes sense. Jabbing yourself with a needle creates a wound. If your body had a choice it sure as **** would prefer not to have a hole put in it. Did you ever do jumping jacks after skinning your knee as a little kid? I'm pretty sure the same idea is at play here. Lifting on a fresh wound probably irritates the muscle even more. But give it a day to heal and it's good to go.
Just my .02