Week 3, day 5 - Mid-week check in

I'm now so confused. I got on the scales this morning for a mid-week check, just to realise that I apparently put on 3 pounds! Not a nice thing to see on the scale, and I thought, this can't be right. I wasn't eating that much, I went to the gym too on Wednesday, now why is the extra pounds? Then I realised that my breasts were quite full with milk, that must make the difference! How am I supposed to get a right number for my weight now? How do I know that I lost a pound because of exercising or only because the baby had a good morning feed? Help.