Fair enough. It could simply be that, as The World said, now that time has passed and emotions have had time to settle, she's realized that she doesn't regret her actions/knows she did what she had to? After all, we don't know how soon after the secret leak FCBD occurred, so it could be that this was the first time they'd met since it happened and if so I imagine Lois was in something of a hectic state of mind.
*Edit: Just a side thought, but I wonder when Clark finds out Lois' reason for exposing him? If he caught off guard and doesn't get an explanation until after he's developed an emotional response to the incident it could go a long way to explaining his inability to forgive her. He'd have to try to reconcile what he's felt for a while with what he's just learned.