What causes constipation?  

Many things. Constipation can be caused by lot of problems. Though usually diet related, other causes are medications (narcotics) or (less commonly) hypothyroidism. The first thing you should do is evaluate how much water you are taking in a day (>8 cups/day) and how much fiber you are eating (25 to 35 g/day). If that doesn't work, look to see if you are taking too many constipating foods like dairy or pasta.