I Have Recently Discovered A S/m Sized, Under The Skin

I have recently discovered a s/m sized, under the skin type of spot. It moves a tiny bit but if touched too much, it instantly feels like a sharp needle being poked through the spot.
I know it is not an sti but it is really painful. And sometimes a bit itchy and sore.
Slightly swollen but not much. I have experienced this before but not as bad. I previously treated it with a hot bath and it cleared up in no time.
However, right now it is throbbing slightly and I am worried. It hasn't changed colour and when closely inspected, it looks like there are a group of tiny white heads arising.
Could you please help me find out what this could be??