It may be that you arent getting enough iron (anemia) or proteins, ameno acids etc - that is key for hair. Its very common for people who eat small amts of food or who 'fast' to lose hair.
what blood tests did you have done. the standard ones arent enough - theres a whole battery of ones for women that need to be done for this. Three types of iron, 3 types of testosterone, FSH, LH, (thyroid) dhea/dheas, Insulin and cortisol, glucose tolerance, etc etc.
You can also have normal levels but be sensitive to something like testosterone and that would make it fall out. Look into PCOS as wel. Do you have any other symptoms - skin problems, oily scalp/skin, acne, difficulty losing weight, any extra hair on body, etc....