I’m at a loss with this one

**edit to add this would be my 3rd baby

I have been feeling a little off and just super nauseous every once in a while a couple times a day so this morning I took a test. Positive. Went to the store and bought a digital one. Positive. But here’s the kicker. My last period started on July 16th. Me and my partner haven’t done anything at all since July 15th. So how in the world would I be getting a positive test if I had a period and haven’t done anything to get pregnant since before my period?? (we aren’t trying so surprise). Also, my period is supposed to come in 3 days.