If she's really thin and her hair's falling out, I'd definitely go check with a vet. Also please get her fixed... kittens are soooooo cute but there are already soooooo many homeless cats in the shelters too... even an indoor kitty may get out, especially when they are in heat she'll do everything to escape and find a nice tom...
i've come to think that cats in heat go through a huge amount of stress, physically and 'mentally', when they can't get out to do their thing... if we're going to keep animals as captive human play things we should at least make them totally comfortable. unexplained weightloss in any animal is usually a sign that something is wrong.
It sounds like she is very sexually frustrated. That explains the loss of hair. But the weight issue doesn't sound good. If she is truly in heat, her body should be storing fat and she should therefore become heavier. Is she a stray? If she is, the best thing you can do is trap her and take her to the SPCA where they will fix her.
~One could always spirit it away to the Humane Society if one can't afford a vet.~I think you'll do the right thing!
