I have IBS and this forum is VERY helpful and comforting.
Undiagnosed IBS can be very scary. But once you know what it is you can just deal with it and that tends to make the symptoms die down.
And yes...you can have the runs and be constipated in the same day. Your bowels are contracting at irregular rates and causing this. If it is IBS stay away from SUGAR and simple carbs (even fruit for a few days.) They cause gas and cramping as the sugars over feed the bacteria in your gut. As I have said before in these forums...my IBS tends to flair up in the summer when my allergies do. I also eat tons more fruit in the summer. In my case Bing cherries (which I love) tend to over feed the party in my gut.
Hope that helps some.