Yes, I had 1 Margarita and was sick as a dog.
I think what you're missing about the above is, if your anxiety is so great that instead of seeing a therapist you felt you had to take a strong drug that alters the way your brain functions normally, alcohol will probably disorient you if you drink too much and ruin your birthday anyway by making you an anxious wreck. The problem is that you just started the Lexapro and have no idea how it's going to affect you yet. I've found that when an antidepressant works, a drink or two is fine even though you're told never to drink when taking them, often it's better than before you were on the drug because the anxiety caused by the disorientation is less. But you don't know yet if the Lexapro is going to make you less tired, more tired, overstimulated, more anxious, less anxious, and how it's going to interact with alcohol. That takes time to learn. So you're on your own here, nobody is going to recommend you go out and get bombed in this situation, so you're just going to have to decide this for yourself and see what happens. Here's hoping it goes well.
I do not take Lexapro, but I have suffered with anxiety in the past. From all that I've learned about anxiety; alcohol is something you should avoid. The truth is, alcohol directly affects your nervous system and blood pressure. Alcohol increases your heart rate and affects your serotonin levels in your brain. Serotonin levels affect your mood; nervousness, and depression set in when the serotonin levels fluctuate due to drinking, which usually leads to anxiety. You might want to keep in mind, that if you end up w/a hangover from over consumption, your anxiety could be overwhelming. Listen to your doctor.