The drug left your body the next day. All ssris other than Prozac leave the body pretty quickly. Any left is too little to do anything. That's first. Second, these drugs don't start to work for most people until 4-6 weeks, but side effects start right away. If you were having panic attacks before taking Lexapro, you shouldn't have expected it to work so soon. If you weren't, it's a good chance the drug was too stimulating for you at the dose you were given -- usually, psychiatrists taper you up on the drug. But they also taper you off the drug, but after only taking it for three days you shouldn't be having any withdrawal effects. On the other hand, these meds affect people very differently -- brains are complicated -- but the symptoms you're describing sound more like the side effects of being on a drug than withdrawal effects. It could be, given you obviously weren't doing great emotionally which is why you were put on the drug in the first place, that you got very scared from the anxiety attack and are having an intensified reaction to that. Know also that withdrawal effects, which it sounds like you're referring to, don't occur because the drug is still in the system, they happen because the drug is gone from the system and the brain is having difficulty adjusting back to working normally again.
About a week... Maybe you should go to the doctor's about the side effects...