Thanx for the query.
I can understand your problem.
Well if your periods are regular and still there is fluttering then possibility might be indigestion
or excessive bowel movements.
I suggest u get a abdominal scan done.
At times worm infestation also causes same type of feeling.
At time multivitamin deficiency
can cause twitching of one group of muscles which can cause the feeling u are having.
But please don't worry if scan is normal try taking multivitamins ,calcium and get deworming done under doctors supervision.
Feel free to ask me question again.