Upset tummy and anal...

So I did anal for the first time today and I liked it until he started going fast..I ended up telling him to stop because it felt weird. It’s hard to explain but like my stomach sort of hurt? Like I had an upset tummy but it only felt that way when he went fast.

Anyone have a similar issue? Tips for next time in how to help that? Everything else was great besides that! It wasn’t painful until then.