during pregnancy your body has to go through many changes, one of which is that the ligaments and muscles etc in your body start to become more pliable and stretch to accomodate the sudden growth.
These can mean that you will get tender areas that are prone to being uncomfortable.
It could just be this, but have a word with your midwife at next appointment. Of course if you suffer further pain or any bleeding then call your midwife or GP.
for future info, your ribs may splay towards the last stage of pregnancy and this can also be very uncomfortable.
Can i also recommend the ivillage pregnancy boards. They are a great way to meet people at the same stage of pregnancy as you and you will find lots of advice and women with similar aches and pains. I joined a pregnancy board in oct 04, had my son in July 05 and our board is still going strong as a mums club.
http://www.ivillage.co.uk/pregnancyandbaby/boa rds/0,,,00.html