2001 Lexus LS430 Question: transmission problem

this car is not driven that often, it only has 85000 miles, only 12000 miles within the last three years. this morning,after starting the vehicle and backing it out of the garage,it was put in drive,and given some gas ,but it was very hard pulling foward. I let the car set for approx.1 hour,and drove it with no problem. the car shifted just fine,just like it always has. what caused it to pull hard?