Don’t get nervous if your friend or a stranger does not seem to notice that you are pregnant yet.
Some women, especially the ones who are into sports or are generally fit, tend to have tight and toned muscles which may hold the baby tight and snug and it may take some time for the belly to push itself up.
If you are tall and have a long abdomen, your baby has a lot of space to grow and the uterus may tend to push itself upwards then outward and thus your tummy may look smaller
Previous pregnancies may have made the abdominal muscles laxer and as such in subsequent pregnancy the tummy may look bigger, sooner than the first one.
But, if you are not gaining weight and if the growth of the baby is also either stagnating or faltering, then please contact your doctor immediately.
Women, especially those who had morning sickness and fatigue in the first trimester tend to not gain much weight in the first three months, but from the fourth month onwards when nausea and vomiting seem to ease, women tend to have a better appetite and begin to gain weight.
Every pregnancy is unique, so don’t compare with others. Body changes and adapts differently to different people.