is it possible I ovulated later?

My LMP was Feb 13. Bleeding lasts for 5 days. According to my two apps I have on my phone it says I ovulated the 27. A conception calculator said I would have conceived the 5-9. We did have sex around this time and after the 27. I was due for a period the 13 of March. I am officially late or missed. I tested the 15 and got a positive and then the 17 and got a positive and then I retested the 18 and 20 (today) and got negatives?! I experienced some spotting the 14, after my expected period. Ever since then I've had killer back aches and headaches. Slight/mild cramping.  Could I have ovulated later??!