My poor little bit

The other day my male cat Was mating my 7mo. Female cat(I think it was in the rear) The reason I say that is the second day after this happened I noticed her hinney looked like it was hanging out it was bleeding a little. I wiped her up with warm water then wrapped her up and laid her beside me and went to sleep. I woke up the next morning and she was dead. Can someone please tell me why she died like that  

Hello!  That's a really sad case and I'm afraid that I can't tell you the reason for definite -  a post-mortem may have been needed for that.   Cats' mating can be repeated and uncomfortable, and it can sometimes be necessary to split the cats up after a while afterwards.  However, this case still sounds extreme - I wonder whether she could have suffered a internal tear, and / or perhaps had an anatomical abnormality such as a lump that resulted in bleeding or rupture after mating.  There are also unrelated causes, for example eating rat poison or a heart problem which could be completely coincidental,  or something indirectly related, such as the female hiding away outside her usual territory and thereby suffering eg a car accident or fall.  In any event, I'm so sorry for your early, unexpected tragedy and wishing you all the best.   The Ralph Site can be a helpful online resource at a time like this.