You say your heart checks out fine. The new thing is the Lexapro, so yes, it could be the new drug. Some people respond to certain antidepressants with a lot of stimulation, so this might not be the right one for you. I wonder, though, how bad is your anxiety and how badly is it affecting your life? I ask because I'm wondering, if you're still functioning okay, why nobody recommended a therapist before trying medication?
Call your doctor. Although your cardio work up came back normal, maybe the medication is not the right one for you. Try going to a psychiatrist and seek out therapy.
Anxiety spikes. your body might need time to adjust the medication or you need to adjust the dose/medication. Good luck enjoy the process of figuring out what works its a long long road... I think an benzo would be better though..