Pooing on carpets

my bt lola is 2 1/2 house trained BUT she will poo on carpets! if the door is open (kids cant remember to shut it!!) she will go up stairs to the bedrooms and poo.
ysterday i was in the summer house doing a work out and she was in the garden i then went out with them and played popped back in to the house for all of 2 seconds and she was in the summer house pooing on the carpet!! I MADE A NOISE AND SHE RAN OUT BUT SHE HAD ALREADY DEPOSITED on the carpet.
she doesnt do it downstairs although we have wooden flooring there and if she needs to go out and the back door is shut she will ask, its been going on for a while if she can make her way upstairs(dont allow them up there as its the kids area only), and we have decking outside .
any ideas why?