What could be the cause of sudden weight loss?

I've been breastfeeding my baby girl since she was born 2 months ago. I started birth control pills 3 weeks ago. I started exercising 4 weeks ago, but didn't exercise at all this week and didn't change my eating habits. I had sex several times the last couple weeks since starting birth control. (it said I only needed to use protection for 7 days) Yet, I lost 5 pounds this week. Seems a little strange. Anyone bored enough to answer?