"I'm coming from China" would make sense if you had just travelled there and were returning home. However, I accept that in that situation you would be more likely to say "I'm travelling/returning/flying from China".
You would not say that if you meet somebody at the airport and he asks you "I saw you yesterday!, where are you coming from now?! (or they won't ask it that way? (I am not English! so I am just asking)