like you said, what was Lori doing already having sex with Shane?? Even if she thought Rick was dead, must have been some kind of love to just jump ship and shack up with Shane. Floosy......have not liked her through any of the seasons.
Lori also slept with Shane in the comics, but from the way it was portrayed, it happened only one time on the road, and she was extremely depressed and despondent and and wasn't in her right mind. It never happened again, but it was enough for Shane to believe that she was his woman now.
I could understand what happened with comic Lori, but yea, TV Lori just comes across as a manipulative bitch, using what she has to assure she has a protector.