When my daughter is overtired, she actually has MORE trouble sleeping - waking in the night crying, waking early in the morning, etc. It may be that you just need to allow her a lot of opportunity to sleep (nap and maybe go down at night a bit earlier for a few days). Once she's well rested, if she is still having trouble, it may be a different issue (teething, etc.)
They also say that children have interrupted sleep when they're about to hit a milestone (crawling, walking, etc.) so it may be that she's developing and growing and that is upsetting her sleep. It could be that she's having some separation anxiety, fears, etc. Could be a LOT of things!
Try to get her well rested, and I am sure she'll get back on schedule.