Gaining weight but not fat?

I'n the last 12 months I have gotten significantly fat, to the point where I've moved up a dress size, however the conundrum is I have not gained any weight. Although I would say I have gotten heavier, although I'm not too sure. But right now, I can feel a double chin, wobbly thighs and back fat, which weren't there 12 months ago. I was 50 kg twelve months ago and 49 kg now. What is this and why? I'd rather be thin with a high weight, rather than fat with a low weight.