Strange results.........hmmmmm

So, I got on the healthy train about two months ago. For the most part, I eat whatever I want as long as it's in my calorie limits. I have also been working out five times a week (two days of hour-long BodyPump - Tuesday and Friday - and three days of hour-long Zumba or BodyCombat - Wednesday, Thursday, and Saturday). Sometimes I go a overboard on food (it's usually on the weekends), but I always get right back on track on Monday. Well, I've lost about seven pounds so far and I weigh myself every morning (I know I probably shouldn't), but I've noticed something weird - I always weigh less the day after I eat a LOT. For instance, this past Saturday, I went to my friend Melissa's birthday party and drank sangria and ate so much that I felt like I was going to explode (I know, not good, but it kinda snuck up on me), but when I weighed myself on Sunday morning, I was 174 - my lowest weight thus far (I started at 181). This has been consistently happening. During the week when I eat as minimally as possible, I usually weigh more when I step on the scale. Is there a logical explanation for this?