What you are reporting may be sleep paralysis. This happens when the normal atonia that happens in REM stage sets in sooner than usual. Those who experience it report feeling as though they are being held down or that they have become solid and heavy. Often other sensory phenomena occur at the same time. Do a little research and see if the information validates your experience.
However — If you believe you may have had a seizure, you should be contacting your doctor immediately, especially if this has happened before. The Internet can be a useful source of information, but it is not a substitute for medical help when something is a serious concern.
I wish you well.