My mum has been going through menopause,not had period for 18 months but had one today?

I would say this sounds pretty normal. Last year when my mom was starting menopause, she would go a long time without having a period and then she would have heavy bleeding for a couple days. She said her doctor told her that it was pretty normal when you start to go through menopause because it takes your body some time to adjust fully. If she is really worried, she should definitely go to her OB/GYN.