Question about ovulation

Yanika • Mom of the best boy in the universe:)
Hello girls,
​I have a question, I am trying to find out my ovulation date and doing opk and also charting bbt, so on Friday and today on Saturday my opk tests were positive, today I also had some cramps.. BUT since I also charting bbt for the last 4 days it's very low 97.25F, 97.21F, 97.25F, 97.25F (36.25C, 36.23C,36.25C,36.25C) 
​As  far as I know there should be a dip in bbt right before ovulation., which I don't have this month.. However opk says O is on the way with positive results. Why?