BBT Charting

Can anyone help me out with interpreting this? I got a positive opk on Cycle Day 15 and assumed that I would see a slight temp dip on CD 16 with a spike on CD 17 to show that I ovulated the day before. It doesn't look like a huge temp shift and is occurring on the same day as I received a positive opk, can anyone explain that to me? I had multiple positive opk' that day (even with different brands) as well as egg white cervical mucus. Thanks so much for your help!