Ok there is something wrong with my very pregnant guppy. She will stop swimming at times and float upside down then start swimming again. She has lost half her color on her tail fin also.
The fish is dying. She has swim bladder infection. The color loss is a sign of illness. What are your water stats? Both these problems arise when water quality is low generally.
Google it. There is not a whole lot you can do once a fish has swim bladder infection. There are medicines for internal infections, but I've heard of little success with these. Good luck.