didnt mean to offend you.
all im saying is that maybe you are over thinking it too much.
if you like A and B was trying to be the matchmaker than let A take you to the party/restaraunt
if you just feel really uncomfortable about going because B didnt invite you themselve, then tell A that. you never know the reason. maybe B told A to invite you, there are a number of possibilities.
sometimes its better just to say something about it, then to over think it because usually that only leads to thinking of negative reasons and such. do you see what i mean? i highly doubt that B intentionally did not invite you. so if you want to go, and A
invited you to go, then you were invited to the party. be confident and go to the party with A, seeing that you are both single and im assuming you want to go.