I'm new to biking, but not to menopausal symptoms. A very heavy flow can be a symptom of peri-menopause (i.e. the time when things are changing leading up to the pause in menopause). I was surprised to learn from my amazingly well informed about female hormones, male, GP, that heavy flow results from a reduced level of oestrogen. I would never have thought that, but it's true. I am 51, very perimenopausal and since being put on the right HRT for me, my previously nightmareish heavy flow which has steadily worsened over the past several years, has reduced.

I agree with the other girls about the blood after a ride. Don't panic, but get it checked out.