I had this too! I did not take the pain meds, mine was caused by the anti-nausea med Zofran taken after chemo. This is what I took daily on a chemo week, believe it or not----6 colace, 2 senokot, milk of magnesia,1 dose miralax, 1 dose lactulose, and prune juice.
Out of my whole treatment constipation was the worse side effect, if it happens again to you, I would recommend the Miralax, it is a fairly new product, and works well, but not TOO well! It should be available at your local drugstore.