Make sure you ask a pharmacist about interactions with calcium and those other medicines (I wouldn't think there are any, but better safe than sorry) and make sure you ask him if all of them are safe should you become pregnant. Even though calcium would appear safe, and even a good choice for pregnancy, there is such a thing as too much calcium for adults, and a fetus may not be able to tolerate you consuming the normal adult dose. Just something to ask about, along, of course, with the prescriptions.
You may want to put off trying to conceive until you can get some control over your diarrhea. I seem to recall that you have lost weight too? Something you will probably want to put back on before you look to having a baby. New information out suggests that thinner women have a harder time of conceiving; it's like the body recognizes that you aren't at prime baby-carrying health and refuses to allow you to get pregnant. That and stress makes it harder to conceive, both for the man's part and the woman's part. That's not to say if you have long term IBS you should never have a baby--there are women on here who have not only gotten pregnant and had successful births, but have even had fewer IBS symptoms while pregnant than before or after the baby. But you should probably wait until you stop losing weight and hair; you're really not at your optimum health. You can always talk to your OB/GYN about what kind of health you should have to give you the best odds of conceiving a healthy baby and having a healthy pregnancy. I don't think there ever is a "perfect" time to get pregnant--there will always be some condition in your life that is less than ideal--but there are certainly not good times to get pregnant too; you should talk to your doctor if now is one of those not-good times. Your OB/GYN is also a good person to go to for tests on vitamin deficiency (although a regular doctor can also order those tests) which, as Lil said, may be the link to your hair loss. Diarrhea will certainly do that to you.
When my diarrhea was bad, I took two calcium pills a day (the recommended dosage if you're taking them for nothing but the calcium for your bones), along with the occasional imodium, although I was able to stop the imodiums pretty quickly. After a little while, I dropped down to one calcium pill a day. If you start to get constipated, take a smaller amount or go to one every other day. I think that three is the limit that you can take in one day, but ask the pharmacist, because, like I said, there is such a thing as too much calcium. And yes, the Caltrate 600 w/vitamin D is the kind that stops diarrhea. I get the generic version of it at Walgreens for less money.