The only reason except for a defective cat is a poor running engine that would plug your cat. find a exhaust shop to have it checked . they dont charge much and if you have to replace it they will be the cheapest.

For a subaru this can be a common problem and can cause the engine to stall or cut out my opinion is get a second opinion from a mechanic i will let you no cats can be very expensive depending on the type and cc of your vehicle