Joey's Girl Found Another Girl's Mascara

We have been so proud of Joey for maintaining a great relationship with his girlfriend for over 7 months now and he almost ruined it all. Is Joey cheating on her?

They were running errands and she puts a mask in Joey's car in the side door compartment. She stuck her hand in to grab her mask and pulled out some mascara that didn't belong to her. Whose could it be? Joey claims he doesn't even know.