Just a quick question about ovulation and af comes early.?

My periods have been really weird lately. I used to be really regular. The 25th of every month to be exact. Well since november it has been irregular. Nov it was the 17th.. dec it was the 17th... jan it was the 10th and then this month it was the 5th. I really don't know where to start about my ovulating. I dont know where i am to begin trying to pinpoint my ovulation with me being so irregular. My doctor told me to count 12 days after my af starts and thats when i ovulate but is this still true if im irregular??? REALLY NEED HELP!!!! ANY SUGGESTIONS???