After period i feel like muscle pain between left armpit and breast. is something to worry about?no lump touchinh. little breasts. no swelling there?

There is muscle effect of progesterone that could cause soreness. It is possible that the progesterone level may be contributing. You may avoid heavy lifting around your period and probably gentle massage of the muscles to help the soreness in the muscles. You may also use warm pack to see if that eases the pain, and taking pain medication is an option.