Is she on birth control? Long term use of bc can cause lower estrogen which in turn causes not so fun menopause like symptoms even at a young age such as dryness and thin skin that could probably make things painful. Are there any unresolved issues in the relationship? If there’s any bitterness or her just not feeling appreciated and in the mood even though she may comply for the sake of the relationship our minds can kind of subconsciously cause the muscles down there to tense up, reject sex, and make things hurt like a stabbing pain. Don’t underestimate a long relaxing massage and romantic foreplay. Sex definitely starts in the mind for women.