Why would one come back positive?

Today I was supposed to get my period and my period comes on time like clock work. I’m a paranoid person so the anxiety in me said to just take a test and it came back positive. I ran to the store and bought more and the other 3 came back negative. What would have caused the first one to come back positive? I read it is very rare for a false positive, faulty test maybe? I am not taking any hormones or fertility meds, I have never been pregnant, never miscarried, I have been off birth control for 2 and half years, my Pap smear came back clear, never had ovarian cysts or anything like that.