When a man ejaculates in there woman so often there vagina starts to smell this is fact , my woman uses a panty liner afer sex to prevent her undergarments fro. Being stained I think most men would suspect that there woman was cheating in your situation however don't jump the gun with out proof . I would simply protect yourself and use a condom
Using condom is a good idea. Another aspect is tiredness after intercourse. If the work she is doing require more movement, she would be uncomfortable after sex. If you find some changes in her attitude towards you, then only you can suspect her.
I'm sure you're familiar with the concept of Gravity. Any liquid that is forced upward will find a way to fall back down again because of gravity. If you don't believe me, take a glass of water and throw the water not the glass up into the air. The water will not stay suspended in the air due to the pull of gravity. Same thing with semen. Obviously it's going to come back out because there is nowhere else for it to go except back out the way it came. It doesn't always all come out at once, and that's why she is concerned about it coming out later at a very inconvenient time like at work. You can completely prevent this problem by using condoms because then this won't happen and she'll have no worries. Since your relationship is clearly tenuous at best, it's really smart to use condoms because heaven forbid you accidentally get her pregnant.
I totally sympathize with your gf and feel bad for her that you're assuming such a horrible thing about her. She likely would be very hurt to know that you're so distrustful of her. Instead of assuming the worst about her you really should be educating yourself better about the female anatomy as well as learning the basic idea of how gravity works to keep everything and everyone from floating off into space.