I finally found the problem. After trying many times unsuccessfully, I received an email at my registered email address asking me to sign up for a Marriott membership account. I thought there might be something wrong with my email address on file. So I used my Marriott Rewards Number to log into the iPhone app and it was successful.

Then I went online and looked at my profile. To my surprise, it showed that I didn't supply an email address. So for some reason, my registered email address was unregistered by someone or by the system due to some bug.

I immediately reentered my email address and then changed my password. I didn't see any points missing or any other suspicious activities. But this worries me enough.