Difference betweeen 12 and 16 weeks pregnant?

Im pregnant and from my last period (which was very light) might have just been inplantation bleeding but i'm not sure. So doctors count it from that period so that means i'm 12 weeks today, but I could also be 16 if that was implantation bleeding. So I don't know if I am 12 or 16 weeks preg? But I cant help feeling more pregnant than 12 weeks. I got a small bump not to noticeable. I haven't had any sickness at all apart from the last 3 days I was sick with yellow bile every night. I get bad heartburn every evening and its hard to sleep for the last 2 months. I'm also tired all the time. Is there much difference between 12 and 16 weeks? Bump size etc?? Do I sound like I could be 16 weeks? Or 12... Please anyone help it's driving me mad!