I'm not a Doctor. but even if I was one, I couldn't help you with the info you have given.
Maybe if you kept a food diary for a few days and posted it on here me or some others might be able to make some guesses.
Anyway, it does sound like something is wrong, so maybe the best advice is to go get a checkup. Checkups are almost always a good idea. Even if there isn't anything wrong with you - it fun to see how healthy you are. also, you can compare check-ups year-to-year and see if you are getting even better. (like cholesterol levels, BP, etc)
I think eating less fiber is exactly the opposite of what you should do. But get your fiber from whole foods, not supplements. Just about everything in a WFPB diet has fiber - so not sure how you can try to reduce the fiber without also reducing how much you eat. Come to think of it, you should if anything try to eat more fiber. Its fiber that helps slow down your GI tract.
There are some things I've noticed that affect my BMs, speeding them up and making them watery. Sometimes when I get home from a restaurant I have a sudden urge to poop and my poop is loose and watery. I have always thought that it might be the bacteria from the restaurant. either too much - or just too strange.
The other thing is beer.
I just looked it up to see why that is and I found this article. There might be some clues in there for you - even if you don't drink.
https://greatist.com/grow/day-after-drinking-stool
