When she was six months old, my daughter had a cyst-like lump near one of her nipples as well which had me a little worried. I called the on-call nurse who first asked if the bump looked red, inflamed or infected and also asked if there was any kind of discharge from the nipple. Since there was nothing except the lump, the nurse replied that they are very common in babies, usually the result of hormones coursing through their little bodies.
We were preparing for a week-long trip so I was extra concerned because I didn't have time to take my daughter in to see her pediatrician before our departure. The nurse told me that the lump would most probably go away and in the meantime, I should not do anything with it. She said that if I still noticed the lump in a month, it wouldn't hurt to have the pediatrician take a look.
About three weeks later, I had to take my daughter in to the clinic for a different reason so the doctor checked out the lump and corroborated the nurse's diagnosis. It is now several weeks later and I haven't noticed the lump again -- I think it's gone... but I'll probably check in the morning, just to be sure.
For me, I felt better after the pediatrician saw/felt the lump in person... so hopefully you have a similar experience tomorrow!