I had the same experience with my first child. Nobody believed I was almost at full term they all thought was only 4-5 months pregnant. Pre pregnancy I was very fit , had a strong core and still swam when pregnant . Was grateful as I have no stretch marks and non eventful epidural free delivery of a healthy 6 lb baby . 2nd pregnancy my tummy muscles not as taut and baby bump size was ‘normal’.
no worries it’ll suddenly swell up during the third trimester as long as you keep healthy and baby is healthy during checkups it should be fine!