Reason For Slight Pain In Upper Chest Above The Breasts

I am 21 5'4" and about 135-140 lbs and I have been experiencing slight pain in my upper chest above my breasts and I'm not sure if its muscular or cardiovascular. Part of me thinks it is muscular because I am now working 30-40 hours a week for my summer job and am at a desk all day. I also have about a 30 minute commute each way, which means I am driving much more than I do during the school year, since I live on campus. When I touch or rub the sore areas it feels better. Also, the center of my chest feels kind of fluttery sometimes, but part of me thinks that might be anxiety because one of my jobs is a lot of pressure sometimes, plus I'm worried something is wrong. Overall, the pain seems to have gotten better in the past week, with a few incidents of pain. When I don't think about it, I don't notice it, which makes me think that it might be anxiety.