can be a pulled muscle , can also be a hyper stretched tendon, deep bruise is always possibility but after a day or two you can narrow a deep bruise down a central location , where as a tendon or muscle is spread cause every movement is felt throughout out the muscle or the tendon as it moves with you, lot of heat with heating pad 15 minutes on 15 off, in between ice to keep swelling at minimum don't stretch or tf or do any thing that requires a lot of muscle action , sometimes a large ace for rib cage wrapped around boobs can help, other than that seeing a dr. really won't tell you much cause unless you get an mri no x ray shows tissue damage anyway, as far as what to tell your Dr. you can easily say you lifted at gym or were putting something heavy up high and felt a pull and soreness, you can also try heat pain patches they get deep in, ibuprofen at night works great cause sadly as you sleep you can't control your turning and might pull more.
