Well, it could be almost anything. What drugs are you on? Any other symptoms? Are they trying antibiotics or what?
What they're probably worried about with pheresis is if this is an infection, they don't want it to become sepsis.
I don't even want to try at guessing. You're in the right place, so try to relax and wait for the experts to figure this out. In the meantime, rest!
Was there anything that could've happened a few weeks ago? Have you had a cut? When was your last tetanus shot? Were you exposed to kids who were sick? Did you feel sick after you ate something? Just try to think if there was anything that could've triggered all of this.
Ecoli is going around but you'd have some GI symptoms then too. If you do have GI symptoms, Clostridium Difficile is always a contender. See, I can't help but try to "guess."
Just let the doctors figure this out.
I really hope you'll get this all figured and will get better soon!