I've read that its usual to feel baby moving between 18 and 22 weeks, for a first pregnancy.

I'm 21 weeks and I've only just really been sure I was feeling baby in the last couple of days. Before that I was unsure if what I was feeling was the baby or something else (because of course there's a lot else going on down there at the moment). I felt something and then nothing for more than a week after that.

I don't think, at 16 weeks, that you have anything to worry about at all. Some women don't feel their babies until more than a month after that, and that is perfectly normal too.