hello there. please please go back to your doctor and INSIST you see a specialist. it is not good enough that the docor cannot find a reason, he or she needs to find someone who CAN. I am sure this is nothing to worry about , but you need to get some peace of mind and an answer to what is clearly not a normal pattern.
my friend felt uncomfortable with her breasts and insisted on no less than 3 scans, on which the 3rd one revealed a growth which was then removed within 48 hours, I am not saying you have such a problem but what i am saying is, LISTEN to your body and if you feel something is out of sorts, PERSIST until you get an answer, You know your body best.