You just brought back some frightening memories!
I had some severe bouts of constipation on the Vidaza/Zofran combo---the worst is when things start and then stop midway. The pressure can be almost unbearable. Once I thought I would have to go to the ER.....I caution about too much "artificial" fiber at once, because that can be bad as well. Fiber from foods, like stewed prunes, oranges, brown rice and cooked veggies are all good, as they container moisture as well. Drinking lots of water is really important and a also stool softener. Miralax is good to take every day, like 1 tsp, for maintenance all month long.
Try not to sit and strain and if you have to get up, and move around, lay down, and most of all breathe and try to relax! Good luck!