thank you! i think this may be the case. LOL
Hi there and welcome. Well, I'll be honest. I think your weight is too low and it is definately messing up your reproductive system. For your height, your weight is such that you must exercise a good deal and eat very little. I was (ha ha, having 2 kids changed everything) very thin as well--- your weight but 2 inches shorter than you and with small bones. I worked out like a maniac and always ate small amounts. wow, looking back it was so unhealthy for me.
So, yes, I think that your weight, diet and exercise routine is causing your period to be very abnormal. Your breast tissue is comprised mostly of fat which you don't have any so they are very small now.
While this won't matter now if you are not in this stage in life, I actively gave up the massive exercising and controlling what I ate after I married and we wanted to have children. I got my weight up to 115 to 118. My periods were more regular after that. And I still looked good if not better. A few curves are a good thing!
So, down the road, you may consider that when you are wanting a child. Until then, I'm afraid that your hormones indeed sound out of sync. You can always talk to your doctor about this to be sure. good luck