I was on SSRI's for GAD (general anxiety disorder) for about 6 months. I tried 3 different kinds, all of which had a generic. From my research (I literally wrote a college research paper on them) and from my experience, SSRI's are a "guess and check" game for the most part. The doctor will prescribe what he thinks will work best, then change it if it's unsatisfactory. It's kind of like acne medicine; there is no universal prescription, different things work for different people.
With that being said I would be a little suspect of your doctor. Ask her if there is any reason she prescribed Lexapro. The usual response is "I have had the most amount of success with this brand". Then ask if there is a generic you can have instead, unless of course there is significant reasoning behind her original decision.
Good luck with your treatment. SSRI's are reasonably safe and work pretty well. Withdrawal is hell, though, so don't use them lightly!