Why are my boobs changing size so drastically?

My boobs have been fluctuating in size like crazy lately!!! One day I will be too small for large C cup bra but the next day I will be bulging out of it. I have stocked up on bra's of all sizes just so that I can accommodate. I'm 18 and in my pre-teen years my boob size was a D32 then as I grew into them and worked out a lot they gradually got smaller and fluctuated between B and C but have stopped for the past few years at C32. I haven't had a growth spurt in about 5 years an I'm expecting one more but I'm not sure if this is my body just growing or if there is something actually wrong with me.