Disclaimer: I don't want to scare you with a worst case scenario, and I'm not a physician (I am a doctor though, and I do work in a gastroenterology department :-) ). There are certainly a myriad of possible reasons, many or most of them probably harmless (including the possibility that it's indeed just your age). I don't know if you had the colon cancer screening by colonoscopy done that is recommended for everybody over the age of 55. If yes, disregard the following.

That said, here's my very recent n=1: I'm 50 now, and I also had some creeping changes to my usual routine and my, let's say, general digestive well-being. I finally had a colonoscopy done, and they found and removed a large polyp from my colon. The pathological analysis revealed that it was well on its way to invasive cancer, but luckily was still completely contained. Two pieces of good news in one for me: my digestive problems were instantly relieved (I was back to what I remembered as being normal within one day), and I escaped colon cancer at pretty much the last minute. Had I waited for the recommended age of 55, I would most probably have been looking at full-blown cancer by that time.

TL;DR: If you've never had a colonoscopy done, schedule one. It's recommended for your age anyway, and can identify (and ideally also correct) a number of different problems.

ETA: Fuck cancer!