Area of density that is not a cyst or lesion?

I had my first screening mammogram at age 39 last week, and the RN at the mammogram center just called to say that they saw an "area of increased density" in my left breast. I asked if it could be a cyst, and she said no, but that it didn't look like a mass or lesion, either. But then she said that digital mammograms aren't perfect, so they're bringing me back in tomorrow for a more detailed diagnostic mammogram.
I am trying not to panic, but of course, it is always worrisome to get an abnormal result. They said they'd have the radiologist there tomorrow to give me the results after the test.
So what questions would all of you ask? And just how worrisome do you think this is?

[Cleveland Clinic]

Dear Kateinwait, It is not uncommon for a density to show up on a screening mammogram.  When this happens, the radiologist will typically call the woman to come back for spot compression where a more specific view of the abnormality can be obtained.  In many cases, the density could be as simple as some tissue that has folded over on itself and will resolve with compression.  However, the spot compression will take a closer look and help to determine if there is an abnormality and whether any additional evaluation is necessary.

[Kateinwait]

Thanks for your response. I have been having pain in both breasts, but especially so on my left side, but it seems to be cyclical (worst in the week before my period.) Having done some reading last night, I'm wondering if this isn't fibrocystic breast condition (disease?) That can cause a density on a mammogram, right?
In any event, I will report back after my appointment this afternoon.
Thanks so much! Keep your fingers crossed.

[Kateinwait]

The density showed up as a 1cm growth on the additional mammogram views, so they did an ultrasound. The radiologist said it's a benign fibroadenoma. They want to follow up in 6 months with another mammogram to see if it's grown.

Thanks for your help!