How Can I Calculate The Next Periods And Ovulation Dates To Conceive ?

Hi, I am off birth control, sort of trying to conceive. Period is 3 days late, I have been brown spotting on and off with tinge of red since Tuesday (estimated period date) have taken two pregnancy tests and they are negative. Today I had diarrhea. My cycles have been normal but last month was the first time I was 2 days late and put it down to stress (moves house and had a bad month) but I've been fine this month. I don't know if flow will come. If it doesn't, when am I supposed to calculate my next period or ovulation date? Could I still be pregnant even though the tests came back negative?