KT,
I have occassional RLS and there's no pain, tingling, etc. like the tv ads say. However, I have the same problem you described. I doubt if Lexapro is the cause. I have taken Celexa on and off, and it hasn't made a difference in my RLS. My mother has RLS, so it may run in families. (Maybe depression, anxiety, etc., the reason for LExapro or Celexa is related to RLS.)
I have found that once I get RLS for the night, I just have to get up out of bed and do something to purposely exercise my legs. Nothing strenuous. Just move them-leg lifts, tapping, or something to get it out of my system. I have to resign myself that I won't get as many hours of sleep as I like and that I'll have to make it up during a nap the next day.
Now, the naps might be contributing to our RLS. I often take a siesta, too, and never have the problem then. I'm always really tired when I take a nap, which is why I take it then, so maybe we have to be exhausted when we hit the sheets at night to avoid RLS! :)And maybe depression and/or anxiety, the reason for taking Lexapro/Celexa make us susceptible to RLS.
RLS may just have started now coincidentally with the Lexapro. Things we get now all started at some point and before we had them we didn't have them. :) Like your car-when it was working fine one minute then all of a sudden, it doesn't!