Backstabbing

I invited a friend to a family dinner and she said that she was too sick to come. Yet later on I look at her Snapchat and she is hanging out with friends. I told myself if she has time to do that then she has time to come to my family dinner. I just don't understand people sometimes. I know that she did not do it intentionally, but still it stung a big. But I validated my feelings so thats all that mattered. However, my capacity to understand peoples actions is becoming very scarce.