as superman said its a precursor to a full pec tear. i explained something similar to my doc after my full tear and he said what i was feeling prior to it were micro tears.

he told me a few weeks off heavy chest wouldve prevented my injury

Do you have any lumps in your armpits? If so, Nitti's comment is applicable.

aw fuck bro, I can remember one specific occasion where this bro we trained with a my first gym was saying this same thing for about a week, he went in on a sat to catch a second chest workout, he was training with my usual lifting partner on the second to last heavy set, "pop" that's it fully torn pec. please bro don't fuck around if you have the ability have an mri done do so, if not baby that for a month or more to avoid such a thing. it's a painful long recovery if this happens.

First sign of a slight pec tear you better take a break and look into TB 500. I tore my pec before and it will be hell for you.

First thing that comes to mind is lymph node. Maybe not even close but if you have an infection from let's say a delt shot, you may be building up white cells in that lymph node.

Pec tendon to the hurmorus likey, take it easy or you may tear your pec. Light weight and stretching and no overtraining, lots of rest.

I had the same thing earlier this summer after an awful pop while doing flat bench. You need to do rehab exercises with really light weight. It took a month for me to be able to lift chest and that was on cycle. Don't fuck with it man or it will get worse, it's either a pec strain or a minor tear. It's awful I know, good luck man.

Tendinitis or a partial tear in the pec would be my guess. stretch, ice. use a lacrosse ball to get in there. don't go to heavy on the bench if it hurts to much. i would go to the doc to make sure i it persists.