DENVER (CBS4) – After a few months of enjoying low gas prices, they’ve shot up as much as 30 cents in the Denver area lately.

What a difference four days can make. Those who filled up their gas tank on Monday saved money. But those who filled up on Friday probably experienced sticker shock.

On Friday Patrick Dehaan, a senior petroleum analyst at GasBuddy.com, said there are several main factors causing the increase. One is Colorado is just catching up with the national average, which is now $3.45 a gallon. Also, refineries aren’t using their facilities to 100 percent of their capacity, mainly because they shut down parts to do maintenance. There’s also the old supply and demand factor.

Overall, gas prices are up from last week anywhere from 15 to 40 cents a gallon depending upon the part of the state. They’re expected to continue to go up next week.

LINK: Pump Patrol: Best Local Gas Prices