When you introduce a new friend to an old friend, and then they forget about you?

[Bee]

So in one of my classes for university, I made a good friend. We'd always sit together, do our work together, and I even got her a free solution manual for the course. Then one day, I meet a new girl in the class, and we hit it off well so I asked her to come sit with us just to be polite. Well now, my original friend, and the new girl have become like best friends. They hardly even acknowledge me now, do the work together.. and I just feel stupid sitting next to them now. If I stop sitting with them they'll think I became jealous of their friendship, and if I do sit with them, well I might as well just sit alone. Any ideas?