Learn Hardcore Russian: Если ты обиделся, нам лучше объясниться сегодня вечером. - If you're upset, we'd better talk things through this evening.

In Russian, обидеться often means to take offense / to become offended, so the past tense обиделся literally means got offended or took offense.

That is why Если ты обиделся is often very natural in situations where English might say if you’re offended. Russian is focusing on the change of state: you were fine, then something upset you.

So this part is not strange at all:

Если ты обиделся = If you got offended / if you took offense / if you’re upset

Why does the verb end in -ся in обиделся?

The basic verb is обидеться, which is a reflexive verb. Here it means to take offense, to get hurt/offended.

Compare:

обидеть = to offend someone
обидеться = to become offended / to take offense

So:

Я тебя обидел = I offended you
Ты обиделся = You got offended

The -ся changes the meaning of the verb, so it is an important part of the word, not something optional.

Why is it обиделся, and not обиделась?