Before you look pregnant, you can still gain weight (especially if you don't have morning sickness). And this early, none of the visible weight gain will be your uterus, it'll all be fat in the places you want it least. Butt, thighs, just pick your favourite place that usually gets bigger if you gain a few pounds.
The problem is, this early you probably don't want your co-workers to know, so you're still trying to squeeze into your old clothes. But you're too small for maternity wear, and there's no way you're going to buy it so soon anyway. But there comes a point when, unless you normally wear loose clothing, your bust won't fit in your tops, and your butt won't fit in your pants, and you have to redefine your 'style'. This is about the point when people start either telling you to go on a diet or asking you if you're pregnant.