What Causes Swelling Under Right Arm?

Had some swelling underneath my right arm that has me concerned. Had been treating for a sinus infection and the doctor took blood and said my WBC count was fine. I did however, go to the womens health clinic for a mammogram earlier this week. They found a couple of very small round white "dots" that they want me to come back in 6 months to re-check. Could my underarm swelling be tied to this, or could it be from weight gain (I have put on a few pounds lately)?