My cholesterol doubled in 7 months

I am a 48 year old woman, about 20 pounds overweight, BMI 28.5. I don't exercise much, but I eat a fairly balanced diet - not much red meat or fried foods, several servings of fruits and veggies a day, mostly low fat or non-fat dairy products.

My Lipid Panel results October 2011 were not stellar but within normal range (and similar to results in 08/2007):
chol - 205
tri - 166
HDL - 52
LDL - 119

My recent numbers are a disaster:
chol - 403
tri - 195
HDL - 75
LDL - 289

What could have caused my cholesterol numbers to shoot up like that in less than a year?? The doctor wants to put me on statins to bring the cholesterol down, but I want to know why it went up in the first place.