Confussseeddd!?

So I took an ovulation test strip off Amazon and it was positive on Wednesday and Thursday (CD13&14) and then it slowly started to get negative everyday so I went and bought the first response ovulation tests to see what it would say and it was positive on Friday and Saturday (CD15&16), for the past couple months I've been ovulating on CD17, has anybody had this happen to them?????? If so, did you ever figure out exactly when you ovulated?! I'm so confused and don't know which one to go by!