An ultrasound of what and where's the mass? If the doctor is suggesting more tests then yes you should go ahead and have them. Ultrasounds can "see" alot of things in great detail, they bounce sound waves off whatever they are "looking" for. As for being concerned, a mass could be any number of things but I guess the thing you're worried about most is cancer. Just get the tests and see what's what. Hope it turns out alright for you.
I have a mass , That needs to be removed and it was discovered on a CT and MRI. Nothing on ultrasound though. These tests don't hurt ..........Good luck