First of all it is perfectly normal to be scared.  Referring you is also perfectly normal. Doing an ultrasound is the normal thing to do. I know the wait is awful.  The fact you are on this site shows how worried you are. Take this one step at a time. You did the right thing and didn't ignore it. The doctor is right to check this out. It's the correct way forward. I hope everything turns out fine for you. 

It’s normal to be scared. But the doctor has referred you for a test. Truly, there’s not much more he/she can do. No-one on this forum can give you any reassurance...those of us who actually have cancer are sometimes waiting for our own results of further tests and it’s hard. I’ve been waiting a few weeks for results, have an operation next week and will have to wait for the results of that, I’m scared. Very scared. But I just have to wait as we all do. I’d love someone to give me reassurance, but of course they can’t. 

I hope your results are good. 3 weeks is not a long time to wait I can assure you.