Hi and welcome to the forum! How crummy it is to be young and have this disease! (I am 57).
Wow, if you are having panic attacks for the first time in your life, that is enough to cause more anxiety right there. First, you have to know that MS can cause problems with your emotions directly. It's not just that you might be anxious about the illness. MS involves the gray matter of our brains and can cause depression and anxiety/panic all by itself, just like it causes pain or weakness. We wrote a couple little articles on this you might like to read.
http://www.medhelp.org/health_pages/Multiple%20Sclerosis/Depression-and-MS/show/752?cid=36
http://www.medhelp.org/health_pages/Multiple%20Sclerosis/Emotions-and-MS/show/753?cid=36
If the anxiety is interfering with your life it is a good idea to get some treatment for it, just like you would seek treatment for pain or dizziness. Sometimes it is enough just to know that these weird emotions are common and don't mean that you are losing your mind. Soooooo many of us have had problems with unruly emotions with our MS.
The Rebif is more likely to cause depression, but depression can have an anxiety side to it also. Be sure to tell your regular doctor and your neurologist about the anxiety/panic. Often they are easily treated. If you don't have an appointment coming up soon, call the office and leave a message about the problem and ask for a call-back.
You are not at all alone here. Stick around and you'll find that for everything you have going on, some of us have had it, too.
Quix