No baby bump yet. Should I be worried?

I am coming into my 13th week, this is my first pregnancy and I notice a little stiffness in my belly low down but I haven't actually noticed any increase in my belly size. I'm a pretty small girl (5'1, 125 lbs) so I figured I would have some sort of small bump by now?

I just had a doctor's appointment last week and they checked the heartbeat, they didn't do an ultrasound so haven't had one since 8 weeks, but the heartbeat was good.

Should I be worried? Online it says around now is when I should notice a slight increase in my belly size.