why was ponyboy sick?

I think you are talking about his time hiding at the church. Basically he had little to eat, smoked too much and was cold.

In Chapters nine and ten, Ponyboy hasn't been feeling well. Unwilling to miss the rumble he takes 5 aspirin (enough to make anyone sick!). After the rumble he becomes disorientated........ due to loss of blood from a head wound. I'm not sure which "illness" you've questioned here, but Aslan took care of the other

Pony was sick maybe from the fountain because is cold and the air.