I am almost 27 weeks pregnant 2 days I lost a little of my mucos plug, I haven't had any bleeding or cramping but I do have some pressure when sitting to long and pain in my thighs. I haven't felt the baby move in about 9 hours but I can hear a heart beat... should I call the on call ob or go to the er. I had cervical cancer last year and they removed 1/4 of my cervix with a leep procedure. My ob has not checked my cervix since I was about 9-10 weeks pregnant. I am just worried but don't want to bother my on call ob on a Sunday.

You should never hesitate to call your doctor if you have a concern when you're pregnant. It doesn't matter if it's a Sunday. CALL NOW!!

If it was a bit of the mucus plug, it can regenerate. It may also have simply been some discharge. Have something to eat and a cold drink and go lie down for awhile as this can sometimes cause baby to move.
Call your doctor as LisaB said, that is what they are there for. You won't be bothering them. They would much rather you call and it be simply a matter of having some peace of mind than to have you not call and it possibly be something they would want to check out.