I am glad to hear that you have had some blood tests run already. Whenever hair loss
is occurring diffusely (all over the head) rather than just in one or a few spots, the first step is to rule out some medical problem. The most common medical problems that can cause hair loss are iron deficiency and problems with the thyroid gland, and these are presumably the blood tests that you have already had done.
If those tests check out, then the likely cause of your hair loss is something called telogen effluvium. This is basically a condition in which the natural cycle of growth and falling out of the hairs of the head is disrupted, so that all the hairs become synchronized with each other and start to fall out all at the same time (normally the cycle is staggered so that you only loose a certain number of hairs per day). Telogen effluvium is usually caused by some 'shock' which gets all the hairs to line up on the same cycle, usually a serious infection or other medical illness
, but also stress and psychological shock can do it. Generally treatment is not needed and the condition resolves on its own, but see your doctor
if you have concerns or questions.