It is possible that the fish ate too much, or that the blood worm had some bacteria.
It’s probably the first one, since the betta fish have a very small stomach and they are always want to eat. They are always hungry and don’t feel the limit while eating.
Also, it is important to clean any leftovers of blood worm from the water.
To help them clean their guts, you can cook some peas and give them small pieces of it.
All the best,