Could possibly be akathisia, could possibly be growing hypomania, could also be anxiety, or something else. You would be best to ask a psychiatrist, bluebicycle. Anyone here can only speculate. Your doctor can get to the bottom of it and hopefully help you accordingly. Are you only on the Lexapro? If so, does your psychiatrist know that? Your psychiatrist needs to know the exact situation to help you best.

I know the feeling (or similar to what you describe). It can be very miserable and grow worse, depending on what it is. I'm sorry you're experiencing this.