Any kind of new bump, lump, or ulcer that you find near your genitalia that is new should always be evaluated by a physician. This type of thing can be seen by a primary care physician
such as a family doctor
or internal medicine doctor
A large red bump anywhere on the body where there is hair grow could be an infected follicle. This is basically where a hair follicle gets closed off and bacteria grow causing a small abscess. These can be quite painful and do occasionally require drainage if they get big enough. The fact that you are monogamous with your girlfriend and use protection does decrease the likelihood that this is an STD, but it does not completely protect you. For example, genital herpes is also a common cause of painful areas near the genitalia. They usually form ulcers, not red bumps and therefore I think herpes is less likely.
Either way I think you should get this thing checked out soon so that it can start to get better. I suggest that you schedule an appointment with your primary care physician. He or she can examine the bump, make a diagnosis and give you the treatment necessary to get it cleared up.