Question

Why are you getting offended in right grammar?

Answer 1

"With", "at" and "by" are all possible. "To" doesn't look right to me. "With" is what I usually hear when the object is a person: She got offended with me. She was offended by something I said. She took offense at something I said.

I don't think "to get offended with somebody" is reflexive, Ivan Ariel.

Cross-posted with Florentia