I've recently had the opposite happen too. After not eating well or exercising for a couple months, I only gained about 3 pounds back. However, I was clearly much bigger and flabby. My pants were/are tight, my stomach is hanging out, and my butt/hips have lost all muscle tone and look larger. So, while it is possible to replace muscle with fat and see only a 3 lb difference on the scale (as I have done), it is also clearly possible to replace fat with muscle and only lose a little weight. I couldn't believe I had only gained 3-4 pounds, because I clearly looked and felt like I had gained much more, like the scale was lying. So, I'm going by my pants now.