I feel very concerned about your post because I always had myself really thin hair and a low hair density. Last year, I lost a lot of my hair and I was really worried about that. I couldn't find why I lost it !
I discovered I had alopecia (Hard to explain because I'm not english-speaking but you can find a lot of informations on google). To sum up, it's the baldness but for women. Hormones were the reason of my hair loss so I thought : my oral contraception ! My hair loss matched when I began my oral contraception. Some oral contraceptions can accelerate the alopecia (I was only 20 and the alopecia affects older women in general), fight it, or have no effect. Since I changed my oral contraception, everything is alright.
If you take oral contraception, maybe that's the cause.
Now I try everything to have my hair back ! Castor oil is really effective, I really recommend it. I know there are some treatments to fight against the alopecia (in France, the name is Minoxidil but it may be the equivalent of "Mens Rogain 5%" which hrcubsgirl talked about) but it's really restrictive.
Sorry it must be full of mistakes in my post, but I hope it is understandable !