Most probable reason for her tits getting smaller:
She's a woman. Breasts can go up or down a size with hormonal fluctuations. I've had gf's where it was like D-cups near their period (titfuck week!) and like B-cups other times.
But maybe you're right. She could also have an eating disorder. She could be a drug addict. These would be reasons to be concerned for anybody, not just hot girls... but I'm not sure breast size should be the only factor in gauging that.