OPK detecting Pregnancy?

Okay,... I have been feeling a little strange, not bad or really different just slight cramps, slightly nauseous for a couple days but it passed, and diarreah.  I am on CD 21 and i am still getting positive ovulation tests? I don't want to test too early, but i heard opk can detect pregnancy? Have any of you ever had this happen? What is you opinion on whats going on?