It could be IBS, it's not uncommon for IBS to alternate between bouts of diarrhea and constipation. But just to be safe, I would call your doctor to be checked out. Not to throw your age in your face, but a lot can happen in four years, given your age. I think another colonoscopy would be in order, but only your doctor can determine that.
I hope you get to the bottom (no pun intended) of your problem.