Weird symptoms, what could be wrong?

Hi,

I have some strange symptoms and my doctor can't figure out what is wrong.

My symptoms are:
- fatigue
- low resting heart rate (46)
- dizziness when standing up
- low blood pressure (100/62)
- nausea and dizziness when heart rate reaches 180-190 (I am a competitve rower so this happens frequently)
- decreased endurance
- frequent nosebleeds
- missed periods (no possibility of pregnancy)
- fainting during or after exercise

I have recently lost weight (I needed to meet the lightweight cutoff) so I am now 5'8" and 130lbs.

What could my symptoms mean?