Lump on arm with blackhead

A year ago, I noticed a small, pinpoint sized blackhead on my shoulder that I squeezed but never totally got rid of. It ended up looking like a minuscule speck about the size of a pore. Recently, I noticed a pea sized lump had developed on the same spot, but underneath the skin. It felt like a hard, tiny not. This week I woke up to a sore,  thumbprint sized lump that as of today, appears like a red bruise above the skin, but feels rock hard and painful when touched. I'm thinking maybe it's a small cyst. Any ideas?