Lump didn’t show up on ultrasound

I’m 19 yo. and I found a lump in my left breast a few months ago. It feels flat and smooth and almost in the shape of a dime. Went to my primary care and was referred to get an ultrasound. Absolutely nothing came up on the ultrasound. They said it was dense breast tissue and sent me on my way. How is it possible that I and my doctor can feel a lump but nothing shows up on an ultrasound? My doctor recommended I go to a specialist but what exactly does that entail and can anything be done if the ultrasound was clear? Do I just wait it out and monitor it to make sure it’s not growing in size? Any advice or personal experiences appreciated!