Why have my boobs shrunk?

So I know this may seem like a silly concern, but it's actually been causing me a lot of insecurity lately. I'm 99% positive my boobs have shrunk. Here's the thing though. I haven't lost any significant weight. I'm about to be 20, so I'm not near a normal age for shrinkage. I haven't been taking any medications or hormone pills. I'm honestly not sure what could be the cause. I am very small built, about 5'3.5" and I fluctuate between 103 and 105 lbs regularly. My weight today is 101 but I weighed in at 104 yesterday. Another concern I have is that I have been showing signs of possible symptoms of endemetriosis. However, after extensive google research, I found no correlation between endometriosis and shrinking breasts. I can't think of anything else that could cause this. Does anyone know what might be causing this? Or does anyone know how I could get my boobs back to their usual size. I'm generally a 30D (and have to wear a 32C because you can't find my bra size.) They definitely seem much smaller now though, possibly a B? Any guidance would be greatly appreciated, because I was already self-conscious of my smaller breasts before I noticed a reduction.