It´s very common for guppies to develop a body like that. The weight of the tail in the case of males, and old females that have bred a lot develop a "hump". It´s also a sign of old age. Guppies don´t live too long, how old is she?
I remember reading somewhere that old females get something like osteoporosis (bone deformity over time).. It's genetic but maybe feed them more variety of foods.
I raised guppies as a kid and they always suffer after a few generations. I always attributed their health issues to them not living in a semi-brackish environment which could be optimal for their health.
Yes, guppies will get like that in old age. You will also see it develop in very inbred guppies, if you are leaving fry in with their parents for several generations. I did that when I was young, and lots of my fish ended up looking like that. I even saw guppy siamese twins once, and they survived to adulthood joined in the middle of their bodies. She could also be sick, or it could be a nutritional deficiency like rickets (Do fish get that?)