assuming your assumption is correct at all, and knowing most girls are into conniving and behind-your-back plotting (except for a select few I know personally :D), shes most likely trying to get dude A to convince your gf to cheat on you with him, thus breaking you two up so girl a can have you all to herself.
if not, there is nothing to worry about though i would be vigilant.
question: on a scale of 1 to 10, how hot is girl A?