It sounds like the condition of your water is off and the fish is not doing well at all. Most Beta fish don't require a very large living space and can do quite well in a very small space. I know people who put their Beta fish in wine glasses. You'll need to go to the pet shop and get a PH test kit for your water. You'll also need to buy some water conditioner for the water.
Change the water in the bowl you have the fish in. Test the PH balance of the water. Add the water conditioner to the water to make sure you remove the chemicals in the water. This is very bad for the fish.
A beta fish needs a cool place to live. If the water gets too cold or even too hot the fish won't do well at all. I made the mistake one of changing the bowl for my Beta and I had to leave. I didn't put the bowl back in the normal place. I left this in the kitchen. The sun hit the bowl and the water got to warm for the fish and he died. So take care where you keep the fish and make sure it is in a good location and the water is the correct temperature for your fish.

Maybe there is something that threatens their life. Don't play with them like scaring them. Let them feel they are in their natural habitat to make them live freely in your aquarium.

Your fish is stressed. Loss of appetite, the tendency of hiding and staying on the bottom of the tank are some common symptoms of stress.

Stress causes of stress are:
- extreme temperature swing
- overfeeding
- frequent water changes
- intimidation from other bettas
- bad water condition

See if any of those factors mentioned above are true for your betta fish and act accordingly.

Check the different nutrient levels of your water, and check if there's a lot of irritants. Your fish might be stressed out. Have you recently installed new lights? How about a change in the food that you feed them? Is it sharing its space with other fish?