I don't know about nail hardener, but nail polish sure makes your nails yellow. It happened to mine during the time I painted my nails black; I used a massive amount of nail polish to wash out the paint almost every two weeks. Maybe, nail hardener would do the same effect.
Or (copied: wikipedia),
-Stains of the nail plate (not the nail bed) are associated with nail polish, smoking, and henna use.
-Yellowing of the nail bed is associated with chronic bronchitis, lymphatic problems, diabetes, and liver disorders.
Anatomy of a nail
# nail plate is what we think of when we say nail, the hard and translucent portion, composed of keratin.
# nail bed is the adherent connective tissue that underlies the nail.
# lunula is the crescent shaped whitish area of the nail bed (when visible)
Maybe it's just the nail hardener. If you wanna be sure, check with your doctor.