I'm not allowed yogurt. Are you sure you are? I've been on plan II, III, and IV so far and there was no yogurt.
Aside from that....here are the possible problems:
Are you measuring your portions? That does matter.
Did you get in your salt and butter?
Did you eat dinner late?
Did you eat fruit after 6pm?
Did you use two slices of bread to make that sandwich? Too many carbs, if so.
Are you pre-menstrual?
Don't fret, though. It's not always steady for me either. In fact, I gained 2 pounds last week and the scale hasn't moved this week at all. I have had weeks where I only lose 1lb and weeks where I drop 5-6lbs. It's not always consistent. You won't continue to lose as much weight as you do the first week. Your weight will fluctuate from day to day, so don't let a 1/2 pound gain bother you. You could have a good bowel movement and lose that much.
My menu could be different from yours because we might not be on the same plan. Mine is also modified to give me more food early in the day because I often have trouble getting in all my food before it gets too late. So I get in one of my proteins at breakfast and split the other one in half for lunch and dinner. Here's a typical menu for me:
Breakfast
3 eggs, slice of lite 40cal toast, 4 oz skim milk, a fruit
Snack
QWL bar
Lunch
1/2 portion chicken over a bed of romaine with 2tbsp Walden Farms dressing
Sm. whole wheat tortilla
Snack
A fruit
Dinner
1/2 portion chicken, steamed broccoli
Snack
QWL pudding