If I'm in the ex-fiance's shoes I would be skeptical as well. Seriously just go reread the first 3 chapters. It clearly stated
- The original owner of MC's body was a horrible possessive person who insistently tried to get that guy
- She poisoned his love saying "If I can't have you, nobody can"
- After doing all that she suddenly turned a 180 and signed the marriage cancellation paper peacefully. Remember people in her family were in disbelief as well.
- Suddenly he saw her at a black market, knowing black market things, and has mad martial arts skills (When she was known to be unable to do MA before)
Could you blame him for getting suspicious?