Problem solved!
This might be interesting for other bottoms with similar size issues so here it goes:The pain from her large cock was because she was hitting the rectum wall.
Our rectums are S shaped but they're different from person to person so if a girl is very well hung or if your S "pattern" is a bit short then guess what...PAIN!
The key is to change positions until you find a way for her cock to slide all the way in without pain so there it is.
At first I was trying to take her lying flat on my belly and that's what caused her huge penis to hit the rectum wall.She took me missionary yesterday and after a while the pain was gone! I could feel it all the way in with no pain at all!
(Many thanks to a friend of hers who's a doctor and actually gave her this advice)