3-4 things come to mind, some of which could be in combination now.
If you're lucky, you just have a bad thermostat that is stuck closed.
If you want to work on this yourself, start by replacing the thermostat and radiator cap. In a Subaru ,anytime you work on the coolant system you must take xtra care to triple check that you have gotten all the air out. It helps to put the front up on ramps or a curb/incline and make sure you run it till the fan comes on. Then make sure there is coolant in the overflow tank for it to draw from when it cools off, then check the level in the radiator again,then cycle it back up to operating temp again and double chek the coolant level in the overflow tank. Check the tank 2-3 more times over the next few drive cycles. Seriously, it's that tricky to get all the air out. If you ever see/hear bubbles in the radiator or overflow tank, you should have an exhaust gas test done on the coolant to check for possible headgasket leak. Too many phase 1 2.5l engines suffer from it. Could be a clogged radiator, feel for cold level/spots on it when engine is at operating temp.
This could be an $8 fix - or $1200 or more. Get it diagnosed soon.
1 Lucky Texan