Why do i keep having this experince?

okay. Every now and then when i go to sleep for awhile i get waken up by something. it feels like something is holding me to the bed and i try to scream for help and stuff but no body hears me and i can open my mouth the only thing i can do is breath heavily sometimes my eyes are open when this happens.. but other times when i open my eyes it stops. i don't know if this is a dream but it really scares me. also it mite be because some times i sleep on the phone with my bf. i have no idea.