I have a hard bump on my chest that has been around for a month and it hurts when I touch it.?

I thought it was a pimple or a bug bite, but it wasn't squishy. It was really hard and I wasn't sure what it was so I left it alone. It started to turn a little darker, but the bump didn't go down. I just touched it and it started to sting and the pain just lingers for more than 15 minutes. I can't find any bite wounds so I have no idea what it is.