That is very, very rare, and even if that happened before, it doesn't mean it will happen again, request a blood test, that will tell you for sure!
I never test postive on HPTs made an appointment. My last child I took 9 HPTs and all were negative. Only to find out 3days later that I was 3 1/2 months.
If you were late when you took a test, it's accurate, if you weren't, take another, it will be accurate by now. Many things can cause irregular spotting, std's, infections, hormonal imbalances, etc. Good luck!!