I've suffered from it my whole adult life. The only doctors who have been concerned were the male OBGYN and interal med guys I saw. My female gynos have not worried about it saying it is natural for my system. I've been on birth control pills for years and that has not produced a monthly cycle. I've been told by several doctors, past and present, it is because of exercising. I'm not skinny nor am I overweight, BMI runs in the mid-20's. I've had numerous tests run, vials of blood drawn, etc and the final conclusion seems to be this is how my body operates and I'm not complaining. My most recent bone density test was great.

However, if you have not discussed it with your doctor you would be wise to rule out other problems. I would not let it go unchecked.