Thanks for your response!
Hello
The most common reason is abdominal muscle pain due to stretching. I suggest you to start with loosening and in a very mild way. With proper acclimatization increase your exercise.
If you still have the lower abdominal pain then you should visit a physician for understanding the causes.
I suggest you to do cardio (walking, jogging, running, cross trainer), swimming whichever does not involve in stretching to a limit which will cause pain.
Take care!