Why did Antony think that Cleopatra died?

You have to remember the scene in Alexandria at the time. Cleo and Antony were defeated, Octavian was about to enter the city. There were going to be consequences for both Antony and Cleopatra. Cleopatra went into hiding. Some think that Cleo deliberately caused a message to be sent to Antony saying that she was dead, and with that certainty, she knew Antony would commit suicide. That way she could try to pin all the blame for the war on Antony. (which according to Plutarch, she tried) Others think it was a matter of miscommunication. All the ancient writers say is that Antony received a message saying that Cleopatra was dead. They don't say if this message was written or oral. If it were an oral message, which would make sense, as the city was in turmoil and a written message could be easily confiscated by the Romans who were looking for Cleopatra, it could very easily have been garbled. As Cleopatra wanted Antony to know where she was hiding, her messenger could very easily have said "Cleopatra is in her tomb" instead of "Cleopatra is hiding out in her tomb". Therefore Antony thinks that Cleo is dead. This makes the most sense.