13 days late

I'm trying to calculate my next period. Last full AF was 8/24-8/28 I had spotting from the 9/6-9/10 on and off only when I wiped I use a liner never caught any blood. So what do I base my last period off of. Doctor says the 6th but it was never enough for a tampon. Done test all came back negative sore boobs hungry all the time diarrhea sore hips and back I'm just clueless