Our office takes part in a race and we were trying to get a team together. One of the other runners in the office suggested a few people who may be interested. One was the woman who sits next to me, who is quite young and thin. After she declined to run, I was just carrying on making conversation, saying something like: "Yeah I didn't think you would want to but..." and then hesitated for a moment. I was going to say you aren't a fatty like most of the people in the office, but I was worried that some of the fatties might overhear this, so I said you are "relatively young." I don't really know why I said "relatively" but this seemed to cause a lot of offence. She then asked me how old I thought she was and I was going to say 30-35, but it felt like if I said 30 she would know I was deliberately going for a low answer in order to cover myself, which may have seemed patronising. So I went with 35, and she said 34. Again, she seemed really put out that I had said 35. Then she said "I hope you get that job in the other department!"
I understand that I chose my words badly, and worryingly I don't really know why that happened. But was did she over-react? More to the point, would you apologise, or hope or assume that she would forget this overnight?