I'm not sure exactly why it is that the Pattersons really tend to suck at reassuring people as regards their intentions but I do know that they don't even realize it. This tells me that the reason Therese is suspicious of Liz is that the Breath only thinks that she didn't do anything to agitate the woman. Given how she had to be told not to make a big, ugly spectacle of herself at Therese's wedding (all the while complaining about the cruel unfairness of not calling attention to herself or upstaging a boyfriend stealer like she was in middle school or some such malarkey), it's quite likely that she made an immature fool of herself and couldn't wait to tell Therese what a bad person she was intruding in her friend's life. It's like watching the title character from the webcomic Candi get all judgmental and huffy because she seemed to think that she had right of first refusal on who someone she was never gonna date had relations with.
This leaves us, of course, with the question of who a Patterson thinks he or she is fooling in cases like this. The answer is either 'no one' or 'just themselves.'