Gastric bypass

Had bypass surgery in July, also had a depo shot right before hand. Started my period in Oct and since then it's been pretty irregular. We're trying to have a baby (I'm 44). How long after you start getting your period from the shot well it take to regulate bc one month it seems good then the next time it comes 2 weeks later than a month after that and after that one 15 days. How do i calculate my ovulation period?