Sex is a joining of two bodies to give pleasure and receive pleasure. It brings two individuals of differing likes, body, immunity and expectations together. This may cause a transfer of body fluids like saliva, vaginal secretions, penis pre cum and semen, from one to another. Along with this secretions some germs may be transmitted from one to another. Infective germs have a different relationship with the body of it's host. If the hosts immunity is strong it may not give any trouble but remain quiet in the body. But when it gets transmitted to another individual, it may cause symptoms. This is called sexually transmitted disease. what you are suffering can be one. so please consult a dermatologist and get treated. The doctor will diagnose and give any necessary medicines. This brings us to the question of what to do in future? Every individual can have germs without knowing much about it. One cannot check all infections. Hence such a transfer of germs is natural among people who come in physical contact with others. This is a common health hazard for those like doctors, nurses, healthcare individuals who work in hospital settings, and they dont shirk work just because they are in an infective atmosphere handling sick patients and their body secretions. So DO NOT label your GF as a "infective" person. It may be so that even you may transmitted some infections to her and she may be unaware of it.