Pristiq and constipation

I have been taking Pristiq for a month now but went up to 100mg in the last week. Since then I have not been able to go to the toilet (for want of a better word). The dr says I should take fiber and movical but it's not working. I was wondering if anyone else had this side effect on this anti-depressant and if it got any better. I'm wondering if I should just go onto something else rather even though it is working quite well for me.

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!