Got a cat for our son for christmas. didn't think about it before hand... if i'm allergic to our cat, could that cause my ms to flare up?

MS flare: Allergies don't usually cause flares in ms. Heat, infection, stress and infection are the usual cause of ms flares.

Unlikely to relapse: But allergy to cat dander will cause possible skin and upper respiratory symptoms. You can safely take anti-histamines if needed. Likely you should do well with your new pet, and have not seen animals ever make MS worse.

Pseudo relapse?: Infection can cause what is referred to as pseudo relapse - where someone with MS feels like they have flared but in fact the MS is stable. It may feel and act like MS but treatment of MS will not help. I suppose something similar could happen if allergies have caused the immune system to be more active.