I did some research on a more in-depth medical aspect and find no bad interaction with
Rebif and Aspirin. In fact the swedish.org clearing shows in its
rebif instructions that one can take up to 650mg of aspirin every 6 hours for relief from flu like symptoms.
Perhaps you just got a weird reaction this time. I sometimes get a bad one from my Tysabri, for example, my last one was accompanied by a headache that lasted almost 24 hrs and then went away. But all others had been ok.