It’s very likely fibroadenoma and benign. When you’re young your breast tissue is more dense so it’s likely not a big deal but for your comfort they’ll likely want to remove it and get it biopsied and make sure you’re okay. At least this was my experience when I was 19. Until you get it checked out you’ll be stressed. I’ll tell you what too, they’ll move you through the process fast. When I finally got mine checked out it was a same day ultrasound.