Hi snoopy,
I found a lump in my breast when I was 25 (I'm 27 now). My first ultrasound detected that lump, and another one. The ultrasound showed they were both solid and had to be biopsied.
When I had my second ultrasound, the smaller lump was not detected. Ater my biopsy I learned I had Stage II breast cancer and the smaller lump (the one that didn't show on the second ultrasound) was deadlier than the larger one!
It's not great that your lump has grown, but that doesn't mean you have cancer. There are many things that lump could be and unfortunately cancer is one of the possibilities.
I'm not trying to scare you, just trying to encourage you to take care of yourself. Get another ultrasound at a different hospital and compare the results. It's so important to ask the right questions and insist on getting answers. If one Dr sends you home saying "You're too young to have breast cancer" find another Dr that will listen to what you have to say, read your ultrasound and spend time on the diagnosis.
It's really scary not knowing -- you'll get through it! Please make sure you get more tests done!
-Kalie