24 y/o F Dxd w/ RRMS. Arm in sun skin turned hot, red, & itchy. Same thing on my chin later but a hive was visible. Super sun sensitive? Allergy?

Not simple. Typically there is not a relationship between RRMS and allergy. However, taking immune modulating drugs (e.g. prednisone) could change that (for better or worse). Are you implying you had symptoms only on one side (one arm) while exposure was bilateral? If so, you might be revealing an autonomic inequality, which might relate to your MS. (PS. If image appears with this answer - not mine.)