Why am I showing already?

I went to get a pregnancy test, but because my period doesn't come every month, the lady couldn't figure out how far along I was. She used the last time I was spotting as my period and estimated that I was 7 1/2 weeks along. But I'm showing already. My pants are already getting too tight, and due to my morning sickness, it can't be because I'm eating too much. My stomach is already sticking out a little and is hard...undereath the little bit of softness ha. Why am I showing already? When do women usually start showing?