If it's mastitis, you would likely have a fever associated with it since that is an infection (and you would need antibiotics for it). If it's a clogged duct, the best thing for it is nursing. Basically, your child will eventually suck it right out of you. But I've never heard of clogged ducts causing something quite that size (unless we're talking a smaller than typical marble). Not familier with breast feeding women deveolping cysts.

Is it a splinter? You'll need to remove it with a pair of tweezers if it is. Either that or you might have burned it or something without realising. Only you will know what you've been up to...

Sounds like the start of a planters wart.

If it's a wart it'll grow bigger. If it's a splinter and you don't get it out it'll get infected. Painful and bigger. I'd show it to some one soon.

idk maybe its like a really small splinter thing,

i get lots of stuff stuck in my hand at the machine shop at school,

if it doesnt go away in a few days just bite at it,

then if all else fails then idk,

deal with it lol idk