Congratulations! How exciting for you! I had a bit of pain quite low down in my belly uring my first pregnancy and it was most apparant when I stood up really quickly. It turned out to more than likely be a combination of ligament pain and also a slight bladder infection as well. Keep up the fluids (reminder to self!) and if you are still getting alot of pain this week I would try to see your GP or carers just to double check.
Good luck!