C.S.
My girls did this and I finally decided it was their teeth. Despite that they were late getting in all of their teeth, they still were teething from 3-4 months on! In fact, my 2 yr. old is doing it now again as she is cutting her molars...but I bet they won't come in for atleast another month of two. If she is chewing on things a lot, it could be her teeth...I would also agree that it could be gas as well.