Tender stomach?

I just turned 13 weeks yesterday and I noticed today that if I press on my stomach between my belly button and pubic hair line my stomach feels tender. I am a bit overweight so I have a stomach so no true bump yet but it also doesn't feel hard at all. Is what I am pressing on my uterus? This is my first pregnancy so I'm not quite sure where bubs is at this stage. I was also wondering when others truly started showing if they were plus sized. I've been told by my mother that she doesn't think I'll show until maybe 25 weeks or so. That seems so far away! Lol thanks!