16 mm follicle size on cd10 doctor said i'd ovulate on cd13 or14 but i had ovulation pain on cd18. i have 31days cycles. can my lp be 18days long?

Ovulation : A woman usually ovulates 14 days before her next menstrual cycle. And so, you're feeling of ovulatory pain Monday 18 and 30 when they cycle makes more sense to me.

Yes: The most fixed part of the cycle is the leuteal phase which is usually 14 days unless you have leuteal phase problems. The follicular phase is more variable. With 31 day cycles you would expect ovulation on day 17 or so.