Sucking in "bump"

I'm 17+3 and I have a noticeable small round bump. I look pregnant. I am 5'2 and started out at 125 and am now 131, so I am small. I can feel baby moving around frequently. However, if I suck in my "bump" it goes almost completely flat. I'm really confused. Is this normal? Does that mean it's not a bump? When I noticed it was very disheartening.