No, don't worry. Here is a few reasons why:
1. If you get a ballon molly they may not be fully grown and expand a bit.
2. The ballon molly could have been underfed and suddenly eat more and look fat.
3. I might be sick and look bloated if it goes on the bottom of the tank give it a salt bath.
3.1 How to do a salt bath?
3.11 Put your fish in a container with fresh water and 1 quarter teaspoon of salt per 1.5-2 liters do this once a day then put in normal tank. Do this procedure once a day every week. If the fish looks better a few days in still do it for the whole week. If it still does not look better do it for another week. If it is still happening on the third week only do it 5 days a week.
4. There IS the possibility it is pregent but look for a really black spot on the bottom next to the tail. DON'T panick if you see a black spot you might be looking in the wrong spot and that could be where they go to the bathroom.
5. Check an online forum that might help you tekk wether it's male or female.
6. If she DOES have babies they will probably eat all of them if you want to save some put them in a seperate container until they grow into adults.
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