This dryness may occur because of a fear or lack of interest in sex, but it can occur even when the woman's quite interested, if she naturally lacks lubrication. One way to ease this problem, of course, is adequate foreplay, but if that doesn't bring the desired response of seat-wetting, try using lubrication. There are many varieties on the market - all you need do is find one that pleases both of you without causing any allergic problems. Vaginal discomfort with sex can also be caused by vaginismus - a spasming of the muscles surrounding the vagina. In some women even the thought of sex causes such tight squeezing down of the muscles that inserting anything, even a small tampon, is impossible.
Some couples never have sex because of vaginismus.
Vaginismus can start with a painful or emotionally upsetting sexual experience. The good experience 5 months ago could have been because you relaxed for some reason. Perhaps you felt particularly close, perhaps you took the time to cuddle and fool around before hand, or perhaps you had a bit to drink or a medication that relaxed you. Obviously, relying on medication or alcohol to have a good sexual experience is not a good idea, but if that's true, it might help you figure out what's going on. The treatment for vaginismus is counseling and relaxation techniques.