Welcome to HealingWell and my deepest sympathy at the death of your Mother. You may want to verify if indeed your Mother had a lump in her breast at the age of 16 just for you own medical family history. I know this is a scary time, but, from what I have learned over the years, it's virtually impossible for you to have breast cancer at age 16; your chance is less than 1 in a million. Yes, it's possible; just very improbable. I applaud you for advocating for yourself and don't stop.
Without a Mom to guide you do you have another female person to go to for female issues that you are concerned about - an Aunt, sister or even your Dr. ?
I suspect your underarm pain was muscles strain or even nerve pain but I am glad to know it has gone away on it's own. Remember I am not a Dr. and cannot dx.
We are here for you so do keep talking with us re your issues or even if you would just like to share how you are doing, we support you.