large bump under the skin

Every once in awhile I get a large knot under the skin (usually on my face but sometimes on my neck).  I shave often but notice that I get them sometimes in my beard, when I grow it out).  They become larger and more sensitive after a couple of days.  The skin around the area becomes smooth with no hair pretruding out.  The area turns red and a large (marble sized) free-moving bump can be visible seen.  After several days, it usually will drain and shrinken,  A large scab will form where the knot was located.  I don't get these often but when I do they are extremely bothersome and unsightly.  What are these bumps and what can be done to prevent them?

Thanks,
Freddy