My breasts appear to be shrinking – what would cause this?

I’m 36 years old and my breasts feel like they’re shrinking. I’ve been a 34C for as long as I can remember, but there’s been a noticeable difference in how my bras have been fitting lately. They are very loose, which I find strange! I just don’t know what could be causing it. I haven’t had any significant weight loss recently, although I did lose about 15 lbs last year and have maintained my current weight since then (about 140 lbs).