Weight gain and belly size at 13 weeks

At my 13 week appointment last week, I had only gained a half a pound since my last doctors appointment which was at 9 weeks. The doctor said she wasn't too concerned about it as long as I was eating when my body told me I'm hungry, which I am. I feel like I'm eating constantly! But I will be entering my 2nd trimester on Thursday and people keep saying "you don't even have a bump yet" "can we see that bump already". I feel as though I can tell that my body is changing but did anyone have this where they didn't show much until one day they finally "popped"?