trying to conceive

So I have not had my period since August. (yes I'm irregular and have talked to my doctor) this happened when I got pregnant with my son. I missed my period for a few months and ended up pregnant but didn't know till I was 7 weeks along. now he's 3 so we are "trying again". has anyone else gone through this? how do I figure out when I'm ovulating if I havnt had a period in 2ish months?