Hi Miss C, our babies are a similar age, and we also had a little stage of "regression" a few weeks ago. DD stopped cooing or smiling/laughing and I thought maybe she was sick. Then it all started up again quite suddenly, and we're back on track.

It is common for babies to have a wonder week at around 20 weeks (5 months) where they regress a little. But if you're worried I would definitely see someone.

Is she smiley in the morning, just after her longest sleep? I always find that the best time to make DD laugh.