What is wrong with my guppy?

She recently gave birth and has been acting and looking strange since then, I can't see any treatable signs off illness, so I haven't given her any medicine. We have a molly who we thought was dying after she gave birth, but now shes absolutely fine, so I'm wondering if it's the same thing, stress on the body maybe? The guppy has been hiding under ornaments for long periods of time, but has been out all day today swimming around funny. Her gills are quite far out of her face but don't appear swollen or anything. Her body is kind of bent so that her tail area is lower down and she's swimming slightly on her side. Please help if you can.