False positives

OK so August 4 I was supposed to start my period. I noticed that I didn't start, so I called my doctor and set an appointment for August 9 because the 7th mark a month without a period, so I wanted to make sure it was after the 7th just incase I was really late. now before August 4 I took a pregnancy test about five days before (first response digital) and I got a positive the day after I took another one and got a negative day after that I took another one and got a negative then I went to Walmart got a cheap $.88 one and got a positive then on the ninth I went to the doctor and got a negative now here it is the 10th and I started my period and  the  pain is unbearable I've never had cramps like, could this mean I'm possibly having a miscarriage?