What Causes Pain Under Armpit Towards Chest?

I m a 21 year old male and I m having soreness, almost bruise feeling pains like under my armpit towards my chest and I feel it on both sides. It feels like there is a bone or something there and it s like sore to the touch and also hurts sometimes randomly. I thought it was from having my bookbag straps too tight while at college, but I haven t worn one in months and I feel it again now. What could this possibly be?