When I started my new, physically demanding job (imagine working out 8 hours a day), the gut I gained during my unemployment disappeared quickly but it was another two weeks or so before the bathroom scale registered the two inches I lost around my waist. There is nothing wrong with gaining a little muscle, especially when that muscle may well keep you from collecting fat in the future.
The problem here is that people especially women look at the numbers on the scale and freak out when they see any number over 100 and/or can’t fit into a size 0 “doll clothes”.
Personally, I consider any woman that you can’t nibble on without chipping a tooth on their skeleton to be more disgusting than a girl who wears a size 26, especially since toothpicks like that are often too busy fussing about their weight to really care about anyone other than themselves.
Just keep eating right and exercising, and take solace in the fact that you will be free of the physical and mental issues that many skinny people have. Malnutrition and neurosis are bigger turn-offs than a few extra pounds, especially if those extra pounds are well-toned muscle.