Did Sylvia Plath have a fever when she wrote Ariel?

It's possible. It can't be ruled out as something that might have been going on with her at least part of that time. She had chronic, terrible problems with her sinuses and had colds on and off throughout most of the winters of her life. In her journals, there are numerous comments about catching, having, or getting over these miserable episodes. Another possible influence is cocaine, which was used (quite legally in those days) to treat the pain of sinus infections.