No bump yet

Hi! I went through 2 successful pregnancies. When I was pregnant with my first, I didn't show much. I started showing at 7 months but it looks like 4-5 months of pregnancy. It's normal cause that was the first time of being pregnant. Second baby, I started showing fast like really fast and I gained 20kg. They say once you gave birth... your body knows what to do already for the next baby. Third baby, I'm in my second trimester and I don't have any bump yet. Is this normal? Mom say that maybe because I'm skinny (I lose all the weight I gained during pregnancy. I'm still 50kg after 2 child) but it's my third baby? shouldn't I be showing fast since my body alr knew what to do?