Sam, it is hard to say. I started Tamoxifen at 31 and now have 8 months left (yea!). My periods have been very regular throughout chemo and tamoxifen, which surprised me. My GYN said that sometimes it has one effect or the other on periods, and it seemed to make me more regular than I was.
I would caution you about believing you are in menopause. You might be, but you might not be. A lot of gals our age get very erratic, but are not definitively in menopause.
Have you asked your GYN about this? Only one time, my period was 2 weeks late, so we did an ultrasound and all was well. I consider myself very lucky in this whole thing.