Well, damn. At least you know you're not chasing a ghost. Lets hope that it's just a leaky O-ring and not major internal wear causing the loss of pressure. By your timeline, I don't believe it's wear. I'm hoping it's just the common O-ring failure and you did good by noticing the pressure and troubleshooting immediately.
I agree with replacing the pan gasket if it's leaking and needs to be replaced anyway and replacing that O-ring while you're in there. But, if a quick diagnosis would help you sleep better in the meantime, do the overfill and nosedive test.