Mastitis, huh? I'd be interested in knowing exactly where you gave yourself this shot in the pec. Honestly, 2.5ml sounds like a lot. I used to inject in crazy places, too. Now it's strictly VL, VG, DG, or delt. Mastitis is inflammation of the soft tissue of the breast...not the muscle. I assume you went to the ER. ER docs put out fires. They don't think too deeply about things like this. They're more worried about the more common ER type things. A guy comes in with a swollen breast and they'll probably run some labs, do an x-ray and, if the radiologist recommends it, a CT....and tell you to follow up with your PCP in a couple of days. Trust me...
To me, it sounds like a classic "bad shot". Heat is probably going to make it worse. I would go in the other direction. Also, I would explore not giving myself injections in places medical professionals shy away from.
But I could be wrong...I'm not a doctor.