You could ask your doctor at your next appointment. It could just be some fat. The body does tuck fat into odd places to store for women who have just had babies, to provide assurance there is fuel for breast feeding. I was told I'd lose weight when I breast-fed my son, but actually it didn't come off until I stopped, my body cached it into all kinds of odd spots against my hour of need. lol
Assuming you are not speaking of a lymph node, right? That's would be pea size. This is bigger? I had something similar and it was a fluid filled cyst. They aspirated it and it was infected (like an infected sweat gland) and went on antibiotics. So, see your doctor!