@tdaniels, it's mostly psychological and can run from mild to Extreme. Mild being maybe just a false positive to a lil belly. Extreme being carrying full term and going through labor to give birth to nothing.
@tdaniels, phantom pregnancy is where your body reacts like you are pregnant but there's actually no baby. Enlarged breasts, lactating, baby belly, whole 9 yards.
Those are really rare. But I know your fear. I was worried up till my sonogram that's what was happening to me. But likelihood is that you won't have it.