What Does A Lump In Breast Indicate?

Hi there. I found a lump in my breast 4 months ago. I had breast ultrasound the other day and was told it is not fluid filled, so it is not a cyst, it is tissue. I was told to keep a watch on it and get it checked/ measured again in six months. if I noticed any changes before then, to get it checked right away. I don t remember feeling this second lump the other day. Is it possible for it to appear so suddenly? it seems bigger then the first.