I also had problems w/ constipation-the rx Miralax which worked great. I had also tried Senokot S & this worked much better. It use to be available only by rx but I recently saw on a commercial you can get it over the counter. Hope this helps!
Also, as Amy posted the weeks after radiation can be the hardest-not to scare you, I just want to prepare you. Just remember, it will pass & things WILL get better. Stay strong...you got this far.