Ovulation usually occurs on the last day of fertile CM but that said many women do sometimes get a second batch a few days later. But I wouldn't rule out later ovulation either. How long have you been tracking since coming off the pill? It will take a few months to get to know your body and learn its own patterns especially with cervical position as this is highly personal to each individual body. I used to ovulate around the 17th so the 18th for ovulation is not uncommon especially if you have a longer cycle. And remember you can get many days of fertile CM leading up to ovulation.