Confused!!

Claire

So I’m 15 days late on my period but my periods are irregular and have been for the last 6 months. My last period was end of June. Yesterday and the day before I did a ovulation test and it’s come back positive but how can this be if I haven’t yet had my period? I did a pregnancy test over a week ago and it was negative. Last few days I’ve had sore boobs and feeling sickly but it passes after a while.