Carol, I know how scary it is to have a breast cancer diagnosis, especially if your situation is a little unusual.  There are at least two reasons I know of that you might have positive lymph nodes and a normal mammogram.  One is that a lump can be on the chest wall, out of sight from the mammogram.  Another is that there is a form of breast cancer called inflammatory breast cancer (IBC) where the cancer cells form in sheets rather than a lump.  Women with IBC often have normal mammograms.  Since you don't mention any other symptoms like swelling, redness, and dimpled skin, it sounds more likely that you do have a lump hiding somewhere on your chest wall.

Because the cancer is in your lymph nodes, part of your treatment will probably include chemotherapy and maybe radiation as well.  You'll need more information about the characteristics of the tumor (how aggressive, etc.) to have a better idea about prognosis.  I don't know if it will make you feel better, but I had 16 positive lymph nodes and a very aggressive tumor profile.  I'm still here 10 years later doing well with no recurrence or spread of cancer to vital organs.  Make a list of questions to ask your doctor and get as much information as you can.  With all the new treatments are available and with no spread to other organs, you have a very good chance of beating this.