It sounds more like it is pineconing - swelling from the inside due to any number of things. Bacterial problems. Bloating. Constipation. The fact she's pregnant is probably neither here nor there - female livebearers kept with males are usually pregnant all the time! I've never encountered one on it's back. So to answer your question - no I don't think these are pregnancy symptoms, I think she's unwell.
If you haven't already, please take her out that tiny plastic box - they're far too small and extremely stressful for a fish that size.