I had similar concerns when I first got my sens. Same issue but if it goes away the next day or so it is just the poly being a pig. But if it persists a few days then I would medicate/feed peas etc.Do some epsome salt as kno4te mentioned. They can go a week or more without eating. It should go down after 3 to 4 days. If not it's some kind of compaction. Belly bump can look like this if a poly is a pig, but disappears always by the next day.
That’s another good option.Look for Rd's treat her thread. Soak the pellets in Epsom salt solution. My son would get that from time to time