I would suggest that you see your primary care doctor
about this issue. They will be able to take a look at this bump and help you decide what is going and and what needs to be done.
I doubt that this is just a 'pimple' give how large you say it is. However, if it is reddened and painful, it could be a boil, which is an infection of the skin caused when bacteria invade through a hair shaft and cause a buildup of inflammation and pus. Many times, boils need to be opened and drained to facilitate healing, which is why you need to see your doctor.
Also, if the area of redness begins to spread, then you might be looking at cellulitis, which is a skin infection as well that causes spreading inflammation and redness, usually without a pus buildup. Cellulitis can be quite serious, and it usually requires antibiotics in order to clear up.
If, on the other hand, there is no redness or swelling, then this might be a cyst under the skin. Cysts are very common and usually not a big medical problem, but if they are large they often need to be removed.
Talk to your doctor as soon as possible!