What Causes Hard Lump On Chest Causing Intermittent Pain?

Hello im a 26 y/o female i have a solid lump in the center of my chest and like angles slightly towards the right. Ive been having pain on n off in my chest for a couple days and i just found this lump. It hurts when i mess with it but then its just like a dull ach when im not touching it now.. what could it be