We had something similar this year. The crack followed the edges of the plywood in the ceiling. We had a foundation company come out and inspect. They took all kinds of measurements and basically said they didn't know what caused it, but it wasn't the foundation. The biggest key is there were no exterior cracks in the outside walls.
They did mention to look out for a plumbing leak (could be under the foundation). Sometimes the ground will shift slightly, not enough to cause a foundation issue, but that can cause cracking. Look for things like toilets and sinks clogging.
They told us to monitor what we had and only fix the ceiling crack because it was so big. How it did in the heat of the summer could make a difference. Well, I didn't fix the crack, but it has actually improved from this spring. Not sure what to make of that, but we'll just keep watching and see what happens.
Anyway, call a reputable foundation company. They won't try to sell you foundation repair if that's not the problem. We've had our foundation looked at 3 times over 19 years and never found a problem.