Ultrasound - sees cyst (very, very obvious) - no worries, no further action required.
Ultrasound - doesn't see cyst - take a biopsy to make certain what it is. Doesn't mean cancer, it just means it's not a cyst and you need to know exactly what it is, or you'll never stop worrying about it. The only surefire way of doing that is by taking a piece of it and looking at it.
I'm not promising a result either way, others have said how low the odds are that it's something that needs treatment, but in itself, the only reason you need a biopsy is because the only thing they can diagnose with certainty through a scan is a cyst and you don't have one of those.
Like if you heard something barking outside, you'd know it's not a cat, but you wouldn't be able to tell without looking at it that it's a male Dalmatian and not a female English Pointer. In that (probably rubbish) analogy, listening to the noise is the ultrasound, opening the window and looking out of it in the direction of the sound is the biopsy.
I hope you get seen quickly.