I've always been slim, but with a rounded belly - I remember pushing it out as a child, pretending to be pregnant to impress my friends. This was the late seventies/eighties and I was never called fat or overweight, but always had a belly. As an adult, if I put on weight, it was only ever on my belly. Occasionally people asked me if I was pregnant. Now I'm 53 and menopausal, I've put on more weight around the middle, but not otherwise overweight. I eat healthily and get a reasonable amount of exercise - gym and regular walks.
If you have a body shape like mine, what clothes do you find suit you? I'm conscious of my side profile. I usually find trousers/skirts and loose (but not too long) tops work best - dresses always look like maternity wear.
Also, have you experimented with your diet? I haven't tried giving up dairy or gluten, to see if it helps, but I sometimes think I should give it a go in case I've had a lifelong intolerance without ever realising it.