Sometimes you can feel cramps during implantation. You would still be having negative tests if that were the case. Still doesn't explain the one positive though.
You could also potentially be very early pregnant and the cramps could indicate a miscarriage. I'm sure that's not what you want to hear, but it's another possibility.
BTW, how long after taking the test did you read it? If it sits around for longer than 10 minutes it can come up false positive.