Lump under skin after being shot with a powerful BB gun?

I got shot with a BB gun (i assume) on the back of my head on a night out when abroad. I was very drunk so don't exactly know what hit me but from details from mates and what I can recall it was gunfire to some degree. Anyway I bled quite a lot and had a huge lump for a few days and then it became quite minute and wound healed. Almost 2 months on and I still have a small, pea shaped lump on the back of my head, near my ear. It's not a solid/fixed lump, I can move it around a few centre-meters in each direction. It begs the question that is this the metal pellet?

I'm not particularly worried about it, just interested as to what it is. Permanent? Due to power of the contact? Certainly a holiday I'll never forget, and maybe a permanent momento haha