It means that you're unfortunate at that time. It's like "today is not my day." "衰神" means the God with unfortunates/bad luck. "我真的是衰神上身耶" = "I must be possessed by the God with unfortunates/bad luck."

就是我很倒霉的意思~

It has the same meaning as "Today is not my day!".

That means bad luck.

It mostly use when you have many bad experiences recently , and to carry some bad luck with you . for example : You just wake up and going go to school , but find out that your alarm was broke your almost late , and after you arrive , you see that you forgot your homework at home ...etc the things happen today are totally not in good ways than you could use the sentence 我真的是衰神上身耶 to tell others how you feel on these events happened