1985 Mustang GT clutch issues

ive owned the car for almost two years and havent ever had a problem with it but have been adding parts ever since i bought it. The clutch was replaced by the previous owner less that a 6k ago and it just started acting weird tonight it goes smooth to about 2" off the floor gets extremley stiff the is fine again what the heck is going on here? Thanks for the help guys im only 17 still trying to learn