Tysabri making me feel worse

I have been on Tysabri for 9 years. I have had RR M.S for 24 years.
Usually when i had my tysabri in the past it used to boost me and i would feel better but for the last 7/8 months it makes me feel worse and wipes me out completely and the fatigue continues without any improvement.
I have had all my levels checked e.g iron, b12 etc and they are all fine.
I told my M.S nurse and all she said was I can’t come off the Tysabri which I don’t really want to do anyway.

Has anyone got any ideas as to why it could be making me feel this way?

Thank you