Frightened. Please support/help.?

I am a 21 yr. old male and have been dealing with diagnosed panic disorder since I was 17. It took awhile to find out what was wrong with me and I was prescribed the antidepressant Lexapro. I felt great for a long time after taking it but eventually it felt like the effects were wearing off. I stopped taking it cold turkey once thinking it wasn't working anyhow and I suffered a moderate panic attack that was enough to scare me into starting up again. After another long while I didn't learn from my mistake and stopped cold turkey once again. Things were fine until yesterday (about 1-2 weeks after stopping my medicine). I experienced a sudden onset of my entire body just not feeling well at all. It was so bad that I convinced my dad to drive me to the hospital. I was in such a state of anxiety that I could not wait and we pulled over and called 911. The paramedics came and every test they performed on me were normal. My heartbeat was a little fast (presumably from the anxiety) and my temperature was a little low (presumably becuase it was cold out). After the paramedics released me saying nothing was wrong we continued to the emergency room. The doctor checked me out and told me that it was 100% anxiety but I just cannot shake the doubts. I told him that I had stopped taking my medicine and he told me to start up again and he also prescribed Xanax to me to be used with Lexapro only when I need it. I have not taken it yet. I've been experiencing headache, stomach ache with extreme nausea, and even sore throat. I want to know if these are signs of SSRI withdrawal from not taking my Lexapro for so long? I've been on lexapro for years and stopped cold turkey for the second time. The first time was nowhere near as intense. Are these physical symptoms from withdrawal? I've heard nausea as a symptom but what about sore throat? And the onset of me feeling ill came on very suddenly. Is that typical of withdrawal also? Please help me, thank you.