I would recommend seeing your doctor and you should add probiotics with your daily regime. I have IBS too and found that the probiotics really help alleviate a lot of the symptoms associated with IBS. Good luck to you!
It's possible that the hormone levels from the birth control play a role in this. It would be worth a review of this issue with your doctor. Have you tried using Immodium D for the diarrhea? There are other anti diarrheals out there, but that's the one I know the name of. I usually have the opposite problem myself with the IBS, so I'm usually making every effort to make it go not stop. I do know that many people rely on Immodium D.
Are you using an antispasmodic medication for when the IBS flares up? If you take one as recommended, it should never be taken long-term. An antispasmodic can help with IBS flare ups, though, regardless of which direction your IBS tends to go.
As young as you are, though, I would seriously consider looking into other types of birth control that don't rely on hormones. Only you know what is right, but it does sound like that if you're asking about this that the hormone forms of birth control might be something worth looking into. Hormones do play a huge role in your overall health, so this is a very important matter to consider. You will read a lot of posts, even here, about the thyroid, because it is the master hormone gland controlling every area of your body. So, it would make sense that while you are checking into this that you get your thyroid levels tested at the same time.