The sentences are 100% correct! :)
I'm not so sure about rules… But for example in the first sentence, if you put the verb (äter) in front of the subject (jag) it becomes a question and you will have to add a "?". "Äter jag inte kött eftersom jag är vegetarian?" (strange question, but dramatically it is correct.
The same goes with your second example. "Ringde jag min kompis?" (Did I call my friend?).