When we say "withdrawl" in my chapter, we mean a permanent severance of ties between the member and the organization. Pin and membership certificate turned in, no coming back.
Since you mentioned a medical condition, did you drop out of school? In my sorority, if you leave your University for any reason (health, finance, transfer), you become an alum and are eligible for (but not required to seek) reinstatement when you return.
I'm not sure how you're eligible for reinstatement, unless this is a case of word choice or a lag in paperwork. Your sorority may have different membership policies than my own. If you are eligible to rejoin to and serious about it, contact the chapter president and the chapter advisor ASAP.
One of the best pieces of advice I can give someone who doesn't quite feel like they fit is to remember that you have X number of NM classes coming, sisters you haven't met yet. NMs are usually very receptive to anyone that reaches out to them, and you might find that you become closest with members of younger NM classes.
If you're willing to put in the effort to find your niche in the chapter, and know that you can balance it with school and your medical condition, do it. If you're planning to just show up and everything will be rosy again, you will probably feel isolated again. Friendships do take work.
I love your username BTW.