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..
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.
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?