Hmmm...I'm wondering if your infusion set has failed. That's happened to me a few times, either because the tubing under the skin has kinked, or I've not changed my set as frequently as I should. I would suggest changing your infusion set. Check the end of the tube when you remove it, if it's bent that's your problem.
Oh, also make sure the connections are all correct. I'm on a Medtronic pump and there have been a few times that I haven't fully snapped the connector back in place after removing my pump. Such a simple, silly thing, but I've chased situations like the one you're dealing with for hours without checking that my pump is connected.