My mum is on this and initially had this problem too...
She took fibre (she doubled what the container said to take) and she used movacol 3 times a day (I think it was 3 times - she took whatever the maximum dose is). She also got some pear juice and drank a glass 3 times a day...
It didn't take long for her body to regulate itself as long as she took what she had to when she had to..
Now she doesn't need anything!