Larger than gestational agen

At my last antenatal appointment, the nurse measured my tummy and she said it's big for 16 weeks. The thing is, I'm not even showing, so I'm like wth. I have a little hump but it's not noticeable at all, she asked if there's twins in my family (not that I know of) and my bf said he doesn't have twins in his either... the doc referred me for an ultrasound to see what's up. I'm kinda scared 😖😱. Anyone else experienced anything like this? It's shocking to me cause I was saying all along and fretting cause I wasn't showing or anything to now hear this. Kinda worried 😣😔

Ps. This is my 1st pregnancy.