Is it just breast tissues? or ..??

I am worried..
I am 27 and have felt a lump in my left breast a few months ago.  I went to my gyn and she said it's 2cm but couldn't tell what it is.  Then I was referred to the ultra sound and they couldn't see anything on the screen, although the two technicians felt it as well.  Then I was referred to a breast surgeon and she said it is likely a breast tissue and with my young age, she doesn't recommend me doing the biopsy.  Should i take her words or go to another specialist?  it doesn't hurt (unless I press it especially a week before period) and it's moveable.   My right sides seems to have it too at the same spot but it's a lot smaller and feels normal.
Should I be more concerned?  Thanks.

[CCF-RN,MSN-JS]

Dear Peach, It is a good question.  You are at an age when this being related to cancer would be unlikely.  An ultrasound is good for detecting cysts, so this being a cyst has been ruled out.  The only way of really determining what this is would to be to take a sample of it.  For your own peace of mind you could get another opinion, again from a surgeon who specializes in breast care.