First of all, if your girlfriend and your roommate used to be boyfriend and girlfriend, then you don't have to be too anxious. As long as you have no virgin complex and no obstacles in your heart, it is acceptable. After all, it is normal for your girlfriend to have such a relationship with your roommate before. You can't abandon your girlfriend because of this. No one has a past, as long as you are not green in the future.
In this case, you and your roommate may be embarrassed, but just don't say it in public, and don't deliberately avoid it with your roommate. Just be normal. As long as there are no obstacles in your heart, you can say anything.
Furthermore, if your girlfriend and your roommate are pure friends, then I advise you to break up with your girlfriend. After all, a woman who can be friends is not a good thing. Maybe you are an honest man, and you are likely to be green when you are together. Moreover, if your roommate knows that you have found a bus, you will lose face. You can pull yourself out while you have just established a relationship and your feelings are not deep.
In this case, if you talk to your roommate, you can talk in a joking tone, saying that you don't know this woman is like this, and your roommate may tell you the story of this woman jokingly.
I have a roommate's girlfriend, who is my roommate's ex-girlfriend. Although I feel embarrassed, both of them feel very good and get along well, because they both know that this girl is a good girl, not that kind of public car.