False positives do happen very rarely. I'm sure you read the directions for the time limit because I know when you're wondering if it's a possibility, you want to do everything right. So long as you read the test within the time limit and not minutes after that time limit was up, I'd just declare it a false positive since the blood test is negative and blood tests are much more sensitive. I'd also talk to your gynecologist and say, "Look, I'm x days late, and I need to know why. If it's not a pregnancy, what's going on?" They may reorder a blood test at a later date but hopefully they will do some other tests in the meantime.