Hi Kelly - I think it's great that you regularly check your breasts, they are yours and you should know them well.
The lump you found could be a number of things, either a cyst, a type of tumor or even a possible lymph node as you said it was near your armpit.
Lymph nodes are bean shaped nodules in your armpits, chest, neck and groin. They help transport nutrients and rid the body of waste. If you have an infection of some sort it is possible that they can swell to a size large enough to be felt.
your doctor will help you rule out what it is by doing a mammo or ultrasound or both.
I suggest taking a sharpie marker and circling the area in which you found it. That way you can track it carefully till your appt. and it will also make it easier for you to show the dr.
Keep in mind that you are bit young and your breasts might still be going through changes - though I am sure you're out of puberty by now, but still hormones do strange things.
Good luck to you, and please let us know if you need further help!
Angi