I burnt my soup :(

Im so sad

I made soup today for the first time using my new pressure cooker. Chopped up all my vege's, I used Carrot, potato, pumkin, cellery, onion, canned tomatoes, four bean mix, chick peas, butter beans, Soup mix & soup pasta. I used 1 1/2 litres of chicken stock(just coverd the veg).

I used the pressure cooker right, let it boil, and when it started steaming, i let it go for 20mins, then turned it off & released the pressure. As it was releasing I started the smell the burning. I took the lid of when it finished and my poor soup was burnt on the bottom :( Its edable (sp?) but the burnt taste is all thru it. Its yummy if you ignor the burnt taste lol :P

Does anybody know why this happened? I really want to try it again (its part if my "get skinny" healthy eating plan lol)

TIA :D