G.T.
Definitely could be some swelling from getting ready to flow, my boobs always got sore and hard spots often right before my period. You are probably just ultra aware of your breasts right now because you just had the mamo.
Give it another week and if it doesnt go away you better make another appointment just to relieve your stress.
Congrats on your negative mamogram, I just had one 3 weeks ago and stressed out till the results came back ;)