Since your pain has come on suddenly, I think it is a good idea to call your doctor and get it checked out. Yes...both an ovarian cyst or an ectopic pregnancy could cause pain. Sometimes, though, it ends up being nothing serious and can never be explained. But, you can't take a chance on letting something go that could be fixed by finding it early (like an ectopic pregnancy). Sometimes pregnancy tests don't register as early as expected even though you are in fact pregnant. That would be a nice surprise! :) I hope you feel better soon!
I suppose it could be either, but I would definitely see a gyno soon. Cysts can act up anytime, but usually they are 'fed' by period hormones and act up worse around that time. Sorry if that's not much of an answer but that's not something that seems to be clearing up.
I am having such problem right now too just like you, Did you ever find out what it was?