Sorry to hear what you're going through..A lot of this unknown crap and waiting around is what sucks. When are you going back in? Try to not worry until then and try to forget anything has happened..I know it is hard but remember that worrying does nothing except wear us out and blow us into anxiety attacks. Do not worry unless you get a bad diagnosis which you are so unlikely to get.
My mother has the dense breast tissue as well and she also tends to get a lot of cysts and fibroadenomas in them as well so often they make her go in more than she would like just in case, but every time, she has been fine.
Trust the doctor, they know what looks suspicious. For breast cancer and anything suspicious in the breast, nowadays doctors are so careful and vigilant that it will be impossible to miss the cancer if you are going in for your regular checks. If they thought anything was really off, they would not let you leave.
I have been in for breast ultrasounds many times and when I'm there sometimes, I have to wait quite a bit longer to be seen because someone has a suspicious node, mass or tissue. From my understanding, they do not let you leave if they see anything extremely awful.
Chances are, you are fine and they just want to monitor it just to be 100%. I hope you feel better