Ever called pregnant, when you're clearly not?

Well I keep getting called it, and I'm getting a little tired of it now.

I fit New Zealand teen size 12 to 14 tops.

People I live with everyday keep calling/saying that I'm pregnant, asking when the baby is due, saying I look really pregnant, offering to buy a maternity dress, saying my clothing is getting pretty tight, and more.

I have thought about buying a pregnancy test just to prove to them that I'm not pregnant and shut them up, but I don't really have the money to waste.

I really need some advice on what to do/say please.