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That makes sense. Being on one SSRI for a long period of time (2007 - present) can be alright for some people, but most patients have to change it up after several years. The body can become accustomed to the drug and it can lose some of it's effectiveness. Prozac and Paxil and Zoloft all work well for GAD too (Celexa is a slightly different formulation of Lexapro, so the others would be better). My hunch is that if you switch for a while, the symptoms will lessen again and you will find that your itchiness goes away. Additionally, after you've been on a break from Lexapro for a while, you may be able to go back to it and it will work well again.
Finally, you might want to talk to your prescribing physician about an anti-anxiety med for the short-term (like klonopin) to get you through the panic (careful though, they're habit forming!)
Basically, in the absence of other major symptoms, I doubt you have lymphoma!