That used to happen to me a lot. After thinking about it some I realized that my mind wandered during peoples conversations because I would become aware of some anxiety and as it blew out of proportion I lost track of what others were saying and had no idea where to jump in so I'd put the look on my face as if I was right up to the minute with them when I really had no clue. often they had no clue what I was doing and went right along talking. It was really awkward and made me super self conscious about being quiet all the time. overcoming many of my anxieties helped with this.

Sorry you're going through that too. Its a bummer.