lymph node pain?

Hi...

I am a 32 year old mom of 2.  I have been breastfeeding my three month old daughter since she was born, and a few days ago I noticed what I thought was a pulled muscle in my arm.  The pain, although not noticeable unless I touch it, is at the top, underneath part of my arm, and in my armpit.  After feeling around for anything different, I felt a small, tender lump in my armpit.  Of course I am fearing the worst.  I have made a doctor appointment, which I will keep even if this goes away.  
I am hoping it is related to breastfeeding... I should note that I have occasionally felt slight pains in my right breast, the same side I have the underarm pain.  I also get frequent mouth ulcers (canker sores), and know from research that infection from those can also lead to lymph node enlargement and pain.
Any thoughts on this or advice you can give would be a great help.   Thank You.

[CCF-RN,MSN-rf]

Dear Mommyto2:  A small tender lump in the underarm area could be many things.  Something as simple as an ingrown hair could cause a small bump and pain.  It could also be related to breast feeding.  Most breast cancers are painless.  However, since you are concerned, it makes sense to have it checked out by your doctor.  It is unlikely that this is from the canker sores in your mouth.  These, if infected, might cause swollen lymph nodes but probably not under the arm.