Hey there new guy.
I have been in your same boat. When people are "using" they will hang out with lower quality people who won't judge them or expect them to be a better person....like that lamer she was badmouthing but sleeping with. I think it also shows she was hurting, missing you. That's why people do that. Whether it's a fling or rebound relationship, it helps ease the pain.
You did the right thing. The pain will ease in time. I myself was hurt for months after a boyfriend who had addiction problems cheated on me.
Focus on you.
giz