Re: anal boil?
The location of this pea-sized lump could be important as far as diagnosis. You will need to see a physician. DO NOT attempt to pop it or squeeze it. You can make the situation much worse, driving the infection deeper.
You could have an anal boil, sure... You could have a simple infected hair follicle which is maybe or maybe not related to the rash. You could have a pilonidal cyst, which is something else entirely. It's impossible to guess based on the info provided -- and only a doc can give you a proper diagnosis.
Get yourself an appointment. Meanwhile, try taking some hot sitz baths. Put a shallow amount of very warm water in the tub and soak your bottom. Be careful not to make the water too hot so as to burn yourself! Soak once or twice daily. When you get out of the tub, dry off the rest of your body FIRST, THEN dry your buttocks area. DO NOT reuse the towel. If this is a boil or anything caused by a staph infection you do not want to spread the bacteria around. Some people find that using a hair dryer on their rump is helpful to make sure the area is as dry as it can be. Again, use a LOW setting and do not burn yourself.
If this turns out to be a boil it will get much larger (usually) and become extremely painful. You won't have any doubt as to whether or not it is a boil -- there's no mistaking it! You'd have two options at that point: get it lanced by a doc or ride out the misery for a few days, taking more sitz bath until it comes to a head and drains on its own. Neither of these are great options, but it isn't the end of the world, either.
Best of luck to you.