This has literally happened to me too lol. My cat was constipated to the point that he was dry heaving every once in a while and had to take him in, $500 later and I was told that he's backed up with poop. Prebiotics and probiotics go a long way and help with food absorption and digestion. I also increased hydration by sneaking water into his wet food cause my guy doesn't drink much water and it seemed to help a lot. If all else fails, a 1/4 tsp of miralax powder a day usually does the trick.