What can I do so people know I am pregnant and not just heavy?

Be yourself. Just be yourself! how you may be perceived by others shoudn't weigh too much on your mind. Stress is not healthy for the mom or for the unborn child.Think happy thoughts. As you progress with your pregnancy, you will inevitably wear maternity clothes which would also tip people off.
Baby on Board. Just be yourself. Eventually the maternity clothes will be the hint.