The same exact thing had actually occurred to me a few days ago. Holding a conversation, reading nonverbal cues and giving out appropriate ones of my own, figuring out undertones in the language, and thinking of what to reply with takes so much space in my head that I don't have any room for anything else. I'm pretty good at multitasking with other things, but I absolutely can not hold a conversation while doing anything else at the same time. I often stammer when I'm talking, especially when I'm nervous, because I just can't think straight when I'm talking with someone (I figured out this is the reason I like to rehearse conversations far in advance). When I've got to wing a conversation or recall several facts to make my point, I fail and sound like I don't know what I'm talking about, even if it would be a piece of cake if I were alone.