It is most likely that, as you suggest, you had a boil or other skin infection. A boil is a generic word that tends to describe infections of the hair shafts, where bacteria get in at the base of the shaft and cause inflammation, redness, and pain. Most boils are self limiting, meaning they don't require specific treatment and resolve on their own. Sometimes warm compresses or topical antibacterial ointment can help.
Occasionally, however, boils can progress and become quite large and collect pus inside of them. In this case they are called abscesses and often need to be lanced or drained by your doctor
. Also, occasionally, the infection will begin spreading to surrounding skin resulting in redness, warmth and pain in a growing area of skin around the initial boil. This is called cellulitis, and it often needs antibiotics to clear up. Fortunately, it does not sound like either of these things happened to you.
The dark spots that have remained in the area where the bumps were is a normal part of the skin's healing process, called postinflammatory hyperpigmentation, and it will gradually fade away over time.
Feel free to see your primary care doctor
if this becomes a recurring problem for you.