You can't tell just by feeling, a mamo and ultrasound probably will be a good thing. I know its scary I just went throught this for 5 months. But lumps can be many different things, fibro adenomas, cysts, plus a myriad of other things. Best to just go forward and find out. Remember most breast lumps are NOT cancer. Hugs to you.
I would have it checked out just be sure, but since most 'lumps/cysts' are benign, I wouldn't worry too much. I'm very pre-occupied with every thing I find in my breasts - although I've always had clear mamograms - I do have lumpy, somewhat large breasts.
My gyn says that if I feel something, and then I can't feel it when I check from another position - ex: standing vs. lying down, then it's 'normal'. She insists that if it's bad, then you can always feel it. Cysts tend to move around - or it's just regular breast tissue. Also, even though they tell you to see if you have the same thing on the other breast - both breasts are not always exactly the same. I know mine somethimes feel a bit different at different times of the month.
Mary