My daughter was completely potty trained at age 2 then she developed mrsa. Ever since she has been having potty problems, is this normal?

Yes. Any stressful event can lead to developmental regression, but these are commonly self-limited and reversible with time.
Regression common. Kids who reach a developmental milestone may transiently regress when they go thru a significant ordeal. A lot of the kids i see with MRSA boils have it on their bottom & the ordeal of emptying an abcess really places them in a wierd place for a while. They don't understand why they got sick so the power they felt being trained may disappear for a while. Be patient & supportive & it should pass.