S.,
You should definitely get this checked out. I woke up one morning this past July with my breast inflammed, painful and about twice the size that it should be. I had a cyst and an infection. They treated the infection first with mega doses of antibiotics, then was sent for a mammogram & ultra sound. The technicians thought there were 2 fluid filled cysts side by side. I went back to my doctor who then tried to aspirate them and learned that they were NOT fluid filled. He referred me to a general surgeon. After 2 surgeries, many ultrasounds and many biopsies, I was fortunate enough to have everything come back as benign. When I first went to the surgeon she told me that on the first visit she was 95% sure that I had breast cancer. Even though yours is probably a fluid filled cyst you need to have it checked. Many women believe that mammograms detect all types of breast cancers, but 10-15% are not evident on a mammogram. Good Luck!!!!!!!!