Ovulation timing

Can ovulation vary month to month? My cycle is very regular, 28 days and lasts 5-7 days. Ovulation usually occurs 2 weeks after period, although my CM was indicating I ovulated around day 18-20? I am still BF DD who is 12 months old, my AF returned at 6months. Trying to conceive no.2. I should be due on the 10th of May. Advice please! :) TIA