BBT Question

I have been taking my temps at the same time each morning and for some reason it seems to not have increased much. I have a 37 day cycle.  I went to the dr to get checked out on March 17 (CD 19) and she said it looks like I'm about to ovulate.  I had a positive OPK on March 18 (CD 20)  I am now 7 DPO and still no significant increase.  Any thoughts on why?  Also has anyone gotten a BFP but had no significant increase?