As someone who has gone through depression, I would urge you to be there for her. From everything you describe, it sounds incredibly similar to depression. Apathy and feeling listless and unengaged are one of the core symptoms. You also know she was taking antidepressants, which in itself strongly suggests she had depressive symptoms. I have never heard of antidepressants being prescribed for the 'side effects of a fever' and it seems most likely to me that they were in fact for depression, but that she didn't want to tell you, perhaps because she feared you not being terribly understanding - you don't really sound it from the tone of your message. Antidepressants usually make these symptoms better, not worse, and I seriously doubt that the drugs (if she is still taking them) would make her listless etc. It just sounds like she has depression, and it can take a LONG time to get better.
In the meantime, a person with depression really, really needs your unconditional friendship. Depression is hard enough to deal with without your friends abandoning you because you 'aren't as much fun as you used to be'. Trust me, nobody wants to become less fun and to stop enjoying things - I am betting this is all a million times worse for her than it is for you.
But depression DOES make it really hard to always treat your friends like you normally would. I remember it was very hard to properly engage and enjoy things, and I really had to force myself to initiate contact ever. Thankfully I had some of the loveliest friends in the world who knew things were wrong without being told and who were subtly there for me when I needed them. They didn't judge when I withdrew or was tired, but realised I was ill (and depression is an illness) and didn't push the matter. Now I am much better and I am so grateful to them. I know I will be there for them if they ever go through something like this, or any other difficult life situation.
Real friendship takes the rough with the smooth, and doesn't count the 'chances' it's given people, or measures how much fun it is to be with people, it just cares. It really sounds like your friend is going through a really, really rough time at the moment, rather than that she is somehow doing this to spite you or anything, and I think it would be really nice if you would just accept that it's nothing personal and that she needs you to stick by her.
It might be a bit more effort for you - calling her, texting her, arranging to meet up - but you can also make it work. If she doesn't have much energy (which most people with depression find), then maybe go round and watch a DVD with her? Or just drop round for a cup of tea and a chat every now and then, or have an afternoon baking. Anything, really, just let her know you haven't forgotten her.