I Lost Weight But I'm Confused!

So I am confused. This week I step on the scale and it read 268 which means I lost 3lbs. I should be happy right? I would be if I understood why I lost the 3 pounds. The last 2 weigh ins I have lost a total of 1.5 pounds. During this time period I followed my diet plan and exercised just as I planned it. The days leading up to the this weeks weigh in I was less than perfect. Friday was my last day at my job so my coworkers threw me a going away party that included pizza, bread sticks and cake and I'll be honest I indulged in everything. This weekend I went a away to my moms house to visit my grandma who has been very ill. During that time I didn't pig out, but I definetly was not following my normal diet. So why did I lose 3lbs when I wasn't perfect and I lost next to nothing when I was? Am I not eating enough calories and my metabolism is taking a dump? I'm going to make yet another tweak and increase the portions of each meal to test this theory. I'll still be eating healthy but just a little more of it to see if this is the problem. We'll see what happens in 2 weeks when I jump on the scale again......