Exhaust Mk3 1.6 diesel focus, occasionally gets really loud

[Imelliot]

so on my focus i have a cat back exhaust that isn't really loud after 4th gear it almost silent but on twice now since the exhaust has been fitted it suddenly became louder and i really do mean louder and was much more of a roar as well and  the car seemed to stutter after 3000 revs in 3rd but any other gear it felt much quicker than before but that could just be my imagination because of the sound. after stopping for 5 minutes to put fuel in when i started it again it went back to normal, So what could be causing this to happen is it anything to be concerned about might seem like a stupid post but im really curious of the reason behind this 

[alexp999]

DPF regen?

[Imelliot]

Never thought of that, how often do dpf regeneration's occur? this is happened twice in about 7-8000 miles. Sounds like that what's going on 

[alexp999]

Regens happen as they are needed depending on the type of journeys.

An aftermarket exhaust is probably amplifying the effect as on the stock exhaust you normally just get a bit of extra burble and the fan will be on full speed.