Answer: It could be for different reasons. You may be having menstrual cramps, you may need to pass gas, or you may be hungry. You can always warm up a towel in the dryer or microwave and place it on your stomach, that helps as well!
Explanation: For menstrual cramps - Take an ibuprofen, make sure to read the bottle!
For gas - You can do some yoga poses to help! Lay on your back and do bicycle legs.
For hunger - Eat haha.
I hope this helps! :D