You could try Caltrate 600 w/ vitamin D. I'd suggest taking one at bedtime and another in the morning, as soon as you get up. It's naturally constipating, so it helps a lot of people with milder forms of D. And, unlike Imodium, it's safe to take long term.
Most people with IBS are worse in the morning. I think it's because your guts go to sleep at night and when you get up, so do they, and they can be grumpy and overactive first thing in the morning. Personally, I don't eat breakfast when I get up; I don't eat my first meal until at least a couple of hours after getting up, if not until mid-day. When I was working, I would take my breakfast to work with me. As I took an hour to exercise and get ready for work, and another hour to get to work, I ate no earlier than two hours after waking up, and I usually didn't eat until about 10am, which was about 3 hours after I got up. That's because putting food on my guts stimulates them even more. But if I wait, they get calmed down and are usually in normal functioning mode before I insert food. I've noticed a definite correlation: I had a bowel movement shortly after getting up (always before I left home for work), but if I give myself an hour after that before eating, that's usually my only bowel movement of the day, or I'd have one more that would be normal. But if I eat before that bowel movement, or immediately thereafter, I usually had at least one other one in rapid succession and it would be diarrhea-like.
Something else you might try is probiotics in pill form. Sometimes diarrhea can be caused by not enough good bacteria in your guts--or too much bad--and probiotics can help people get back into balance. They're safe to use (no drug interactions, no overdosing) and the only known side effect is gas, which, like fiber supplements, has a tendency to go away after you get used to them.