Temp Not Going Up--No Ovulation?--Help!

My cycle buddies and I are stumped. Here are my charts. I got a not-quite-positive opk on the morning of CD13 (3/27), and a positive opk that evening. I had a VERY positive opk on CD 14 (3/28). On the morning of CD 15 (3/29). OPKs were back to negative. My temp dropped on CD 13, but hasn't jumped up above my other temps since then--it's just gone back to where I was pre-o and has not stayed elevated. Did I not ovulate? Is there something I'm not thinking of? I'm taking these temps at the same time every morning with the same thermometer, and I don't move or get out of bed before taking my temp.