Engaged?

This morning my S/O said he had a surprise when I got out of the shower. Well I go in our room and he hands me a small box. Inside is a beautiful simple ring. I told him I loved it and put it on. (My ring finger) I assumed it was a promise ring because he didn't say anything when he handed it to me. Well later while we were out he casually brought up "now that we are engaged..." I'm afraid I missed something. He didn't ask me? Yes I want to marry him but he didn't tell me it was an engagement ring. I'm not sure how to feel.