abnormal mammogram

Hello!  I'm very very scared.  I received an abnormal mammo letter in the mail.  I called my doctor and was told that both breasts were heterogeneous (which I guess is very dense) and that the left breast was asymmetrically dense.  I have a follow up mammo and ultrasound tomorrow.  I do not have any pain in my left breast, but when I lay on my stomach I almost feel that the left breast is "different"...not pain, but different.  I'm 45 years old.  My last mammo was in 2011.  I have two children that are 17 and 6.  I'm so scared and can't think of anything else.  Has anyone else had this. 

Thank you!! It's just so hard not to think the worse. I think it bothers me that I can tell something is different with my left breast and noticed "something" about it even before the mammo, but I don't know what that something is. It for sure is the bigger of the two, but nothing way out there.