Hi, Vivian. Unfortunately, all houses settle a little. And since this is an entry door, somebody slamming the door or using a little hip to open it could be causing the wall to move a little too. So the chances are good that you don't have a major structural problem. You will probably be able to spackle the crack the next time you paint and forget about it.
Having more information like the age of the house, location, and kind of foundation might give us a little more help in guessing. But without a thorough review of your house and what is going on with it, there is no way to tell.
If you fix the crack and paint and the crack comes back, or you get a lot more cracking in different areas, I would not be too concerned about it.