thats happened to me before.
im not on imuran so i dont know if that is it or not.
my guess would be that its a mix of everything.
meds, UC, taper, just feeling sick in general, loss of blood (if applicable), eating changes, dehydration....
could be a whole heap of things separate or together that just make you feel crap.
i have had bad sweats at night and its winter time here. freezing.
i think that is the pred.
sorry im not much more specific help, but i reckon its the whole lot coming down on you and affecting your body.