I stop menstruating in September when I was having radiation for rectal cancer (ovaries were getting zapped too). I am now going through chemo and I still haven't got my cycle back and suffer from hot flashes.
My GYN told me to give my body 6 months chemo/treatment free to see what will happen. Then if nothing happens, we would discuss my options. I would give it a good couple of months for your body to get back into it's own rhythm.