You are doing fine! You haven't had a whole week (Weds to Mon), and 2-3 lbs is great for starting out. Remember that weight fluctuates, that's why it's best to only record once a week. There are lots of stickys that talk about this around here. You don't mention how much of a change this new eating pattern is, if you were following something not to different, then the change isn't going to be as great. Leave it another week and see how things go.
The only thing I would imagine your vitamins could do, if anything, would be to alter the amount of water your body hangs on to. But I wouldn't expect that.
Some people do lose weight rapidly on the beginning South Beach stage - but that rapid loss is often ~8lbs or so in the first two weeks. By losing 2-3 lbs in 5 days you are well on the way to that. Longer term (averaged) you want to be looking at 0.5 to 2 lbs per week for safe, steady loss. Some weeks I seem to lose 3 lbs, others nothing at all (not counting off-plan times). Stick with it - it's nice to see a big boost at the start, but not essential. Don't get discouraged,