sounds like could be alopecia like BG pointed out but not sure if can in so young (my Mum has had alopecia for many years, to the point of no hair at all ... at least there is some level of help these days unlike it was for her many years ago in the 1970's).

... keep in mind what BG has suggested too, anything could be possible and sounds like you are keen to cover all grounds !!

(BG ... i didn't have alopecia but did go through a stage of female pattern baldness (patchy hair, though i can get alopecia in the eyebrows) about 7years ago and using prescribed solution for it which worked but sadly i didn't know i was in the early days of pregnancy and therefore resulted in m/c ... no hair is worth the loss of a fetus/bub but for me i didn't know at the time and wasn't warned by the specialist ... So glad your holding off with it there for that reason.)