I was bitten by something, probably a spider about 4-5 days ago. At first, it was a little bump, similar to a fire ant bite. The next day, it developed into a hard lump under the skin. Now, there is a white-head looking thing on top of the lump where I was bitten and light colored redness all around it. The area feels warm and is very tender. What do you think it is, and what can I do about it?