In this context, you would use the word lump. As it would stand for a bundled mass, that has been shown on an ultrasound. The words bump and lump can both be used, however, to mean something similar. I have a bump/lump on my head. In American English at least, it is common to say bump, however, bump is also a verb. Lump can also be used to describe something that is under the skin, or in the body, and has a hard knot feeling. There are many websites, such as WebMD, you can search for, for medical advice and questions. However, I caution doing so, because sometimes the meaning of symptoms could be nothing, but looking it up on a website, can make it seem a lot worse.