I dont think so, i believe they ussually loose weight as they are ussually more active, trying to find a mate, being uncomfortable, hormones etc...
It is probably just a coincidence. are you going to breed her? what breed is she? I would get her spayed asap. I would limit her food intake & give her more excersize.

it may look like she is a tiny tiny bit chubbier but only on her belly as their mamary glands swell a bit when in heat.