Need to know what happened 😅

Hi this is my first time needing to ask something like this but my lack of experience has caught up to me. I (18m) and my bf (also 18) were having some fun the other day. I’m the bottom and it was all great. I’m not completely used to anal yet and not familiar with all the sensations associated with it.

At one point while he was fucking me i started shaking violently, as if I was orgasming. But I didn’t actually cum. I needed to take a brake from being to sensitive as if I did orgasim, so that what me and him settled on. I’m not entirely sure tho. It’s odd to me that I could “finish” without actually cumming. I would like some perspective on what happened.