Cum pouring out

Help! So every time after me and my boyfriend have sex and he cums inside of me the cum just pours back out! It's super didgusting and annoying and I don't remember it happening when we first got together, Why does it do this and how can I make it stop ??? 
Follow up question, how do people do it places without making a mess ? I always need to have a towel near by so I don't make a mess lol