True, but I think this was due to a frustration that had been building up inside Lee for quite a while.
She's been in danger quite a few times (some indirectly because of Jim, others not), had a miscarriage, and after taking Jim's advice to move on while he was still in prison and finding happiness with another man, he comes back to the fold and she begins to get a little wary. Plus, can't imagine she was all too happy that Gordon verbally chose to "sacrifice" her for Valerie.
All that escalating to her wedding day where Jim comes in and admits he's still in love with her despite her telling him to move on already. She was pretty much starting to get sick of him. And when Jim shot Mario, in her point of view, that was the breaking point.
While the execution wasn't the best, the progression and growth of Lee's dislike of Gordon was there, and wasn't as much as out of the blue as you realize. Those are my thoughts on it, anyway.