hair loss?

I have been noticing that my hair has been getting thinner and more brittle for a while now. I have naturally thin hair to begin with so I've been using Paul Mitchell's Volumizer and was very happy with it for about two years, until I began to notice this change in my hair. I'm not sure if it's related or not--has anyone else experienced anything like this? The only other thing I can think of is that I've also been taking erythromycin for almost three years for skin problems, but I don't think erythromycin is known to cause hair loss. Please post ideas! Thanks.PS- I can't tell whether I'm actually losing hair or whether it just feels thinner--I haven't noticed more hair falling out than usual, but it definitely feels much thinner.