During sex, i bit my girlfriends lip quite hard. her bottom lip began to swell and go red, she's afraid it is a cold sore? what else could it be?

It's a "hot" sore: It sounds like the lip is injured and possibly infected from "hot sex" not a cold sore such as herpes. If it continues to swell and be red, she should have a MD or DDS evaluate the area as the mouth has the most pathogenic (disease producing) bacteria in the body. Yes, even more so than the vagina.

Trauma, infection..: A human bite is quite painful. The redness and swelling can be from the trauma of the bite. If you broke the skin, it can also become infected and the infection can cause redness and swelling. If this just happened, you might want to put some ice on it. Ibuprofen can also help with the pain. If the skin is broken and it looks infected, she will need to be seen by a doctor to get antibiotics