You may not have changed your eating pattern or diet. But that doesn't mean your body isn't changing on you. It either sounds like you may be starting to be lactose intolerant or starting to have gall-bladder problems. You may try giving up dairy for a while and see if that helps. And you may want to avoid greasy foods. Or certain types of oil. I have found that some oils are worse for me than others. I would suspect that it starts at about 2pm because you now have two meals in your system and are up and about. It just may take that amount of time for your body to "move things along" and really gets fired up about that time, until it is time for bedtime. And things slow down again for the night. You might want to check out a gallbladder flush on line to help with gallbladder function. I would definately document what you eat. Even with no changes in your diet, it sounds like your body just isn't tolerating something. And it could be miniscule whatever it is. The other posts listed some top offenders. Like fiber, salads, new multivits, broccoli, high fat foods, fried foods, greasy foods.etc. I would also make sure you aren't constipated. That sometimes makes extra gas in the gut as the gut is trying hard to "move things" along that don't want to move quickly enough.
Hope you feel better soon,
L.