Try some brown sugar and water.
Mix a teaspoon with 50mls water. Give in between feeds until poos are back to normal. Always has worked a treat with my kids.
If that doesnt work (which it should) you can try prune juice. Just mix 1 teaspoon of prune juice with 1 - 2 tablespoons of water once a day.Once poos are back to normal then stop the above.