From my own experience and from reading a lot about Crohn's, fatigue is just a standard side effect. As is achy joints and numerous other issues. Doctors can't pinpoint what is causing it or exactly why it happens, but it does. I go through fases and when my intestines are showing active inflammation is also when my joints are most achy and I'm most fatigued. When the inflammation calms down, the rest of me seems to recover too. And it has taken me many years to come to terms with the fatigue. As there is no physical reason for it, I always feel guilty like I'm being lazy when I can't seem to get up and moving for a day or two. Sometimes it is just lazyness, but most times when I feel so exhausted I can't move after just a short walk around the block it is the crohn's telling me to slow down and get some rest. It does eventually pass and the energy slowly comes back. Then when it does, go out and enjoy it!