Cherry Barb

Cherry barbs are egg layers so strictly speaking they don't get pregnant, they get full of eggs. It is possible for female fish to develop eggs in her ovary without stimulation from a male; she could either expel them - and without a male they can't then be fertilised - or she could reabsorb them.

Other possibilities are overeating as you suggest. If you don't feed them for a day, if this is the cause her belly will go down.
Or constipation. Feed cooked, deshelled, chopped up peas instead of their normal food then miss a day before feeding their normal food again. It will do all the fish good. Remove any uneaten pea before it goes off.
The last possibility is dropsy. Do her scales lay flat against her body or stick out like a pine cone? Looking down from above is the easiest way to see this. Dropsy is not a disease in itself, it is a symptom of several other things from internal infections to parasites. These cause the kidneys to fail and the fish then retains fluid in the body causing this swelling. Once it reaches this stage there is not usuually much that can be done, I'm afraid.

My female cherry barbs are fat, too, but they seem happy enough. I did see them disappearing into the bushes with the boys, though.