Girlfriend recently feels much less pleasure during sex, but feels pain, which actually takes her off the mood. she is 19, sexualy active for a year?

Dyspareunia: if the pain is upon entry then possibly is due to lack of lubrication, trauma, inflammation, irritation and infection or due to Vaginismus, if the pain is deep then Pelvic inflammatory disease, Endometriosis, prolapse of uterus and etc. Make sure that your girl is emotionally available, has no stress, anxiety or depression. Make sure your relationship is solid, support her, see an Ob/gyn