CHILL!!!! I had this problem when i just deliver my son. I was like you! I told my husband, could it be tumor.. I found it under my armpit too. Until I went to see my Gynae, she said its milk duct. Use warm water with towel to massage the area for about 15 - 20minutes. So the milk duct "melt" It wont go away if you do it 1 time. Continue to do everytime before you bf your baby. Massage with warm towel. I know its really troublesome. This is the only way to melt the milk duct.
Good luck! And dont stress yourself too much. =)