Why doesnt my dog want to walk anymore?

my dog is a three year old 80 pound mastiff mix.i have been taking her on 30 to 45 min walks everynight and as of a few days ago she will not walk more than 5 or 6 houses down and then does her buisness and wants to go right home.i dont understand what the problem could be,sha was walking fine and all of a sudden,we get a few houses down and sha stops dead in her tracks and wont budge! anyone ever had this problem before?please help she needs her exercise.i usually try to take her at night when it is a little cooler but she does this day or night.