Thanks for replies. I've been having a look around on net about SPD, and that may be what it is. It seems to to fit a bit better than round ligament pains. The 'pain' is very, very mild - more a discomfort than a pain, and doesn't occur too often, only after a particularly active day. I'll talk to the midwives about it next time I'm in.
From the sounds of it, SPD is pretty common, just a bit painful and uncomfortable. I'll find out what I can do to limit discomfort when I talk to the midwives.
Thanks for your replies - I had no idea this condition existed.