hi escargo.. maybe take a photo of it.. i am sure there's lots of sifus here to give advice..
or maybe you can do some check list..

1- is the crack caused by water leakage from toilets/roofing?

2- any strong movement from repair works eg: stepping on ceiling whilst repairing pump or water tank?

3- if possible use a torchlight and peek through the manhole to see if there's any loose screws on the suspension system..

good luck...