[Soundy]

The fact is, we can't diagnose this over the internet with a few pictures. You really need to have a GOOD contractor come and take a look at it... at the very least to confirm the actual construction of the floor and foundation. He doesn't necessarily need to take core samples or anything extreme without at least first checking on some of the basic things that have been mentioned - whether it's a singe-pour foundation, whether the proper rebar was used, and so on.

Everything suggested is a possibility, from a shifting foundation to just lazy tile guys, but nobody can tell you that just from posting on a forum - it's something that has be inspected IN PERSON.