I've been in the same boat before. It stinks. Your main problem here is you don't have a signed contract...The simplest thing to do is tell the G.C. you need to go take care of other jobs,and you may be off site for an undetermined amount of time. they will either expediate paying you or there will be some other guy there the next day. either way... your not giving away your services. The other thing that others have touched on here. Alot of larger companies don't pay the day they recieve the invoice. I've heard of a 90 day period. I personally would not do any more work for them, because i o u's don't pay my bills, not sure about you...
