What could it be?

Ruby
When having sex with my boyfriend
Especially doggy style i get this urge feeling that i need to pee and it gets so strong that i pull away 
Idk why that happens because i have never felt that before but am i about to have an orgasm? 
The only orgasms i had with my ex boyfriend were with clitoral stimulation 
Also will it actually come out as if it was pee?