Anxiety induced stomach aches only when trying to go to sleep?

Ive been getting stomach aches at night lately when I'm trying to fall asleep. It doesnt have anything to do with the time or laying down, because I can stay up until 3 in the morning laying on my bed watching tv and be totally fine. But when I consciously try and go to sleep, my stomach gets all tight and nervous and I get a stomach ache and feel very nauseous. Does anybody have any suggestions as to how I can overcome this and get to sleep, or why this has suddenly started to happen. Thank you very very much. =)