I actually felt physically ill when...
... The implied rape was about to happen. Here you have this character in Selena who's extremely strong both physically and mentally. She's serious, focused, self-sufficient and smart. She won't hesitate to kill if it means saving her own life. She is literally the opposite of the damsel-in-distress. I feel like that started to wane the longer the movie went on, especially after Selena is consoling Hannah over Frank's death. She becomes more of the love interest of Jim and gets visibly shaken after Mitchell takes her machete and grabs her.
Then came the impending rape when Farrell and Jim are locked up and then taken away, and I got really uncomfortable. The fact that you have this really badass, strong-willed female character like Selena that's about to become a rape victim left me really unsettled. I assumed it was happening when Jim was locked up (I honestly don't know why they had to wait) though luckily it did not. At this point Selena's whole character just falls to pieces. She still shows her crafty side by at least formulating a small plan to help Hannah cope with what's about to happen, but she still gets dehumanized by the army guys and treated 'like a woman' - manhandled, ogled, verbally abused and put in a sexy red dress.
Now she's the damsel in distress - as she has to wait for Jim to save her - and it just felt really aggravating and wrong. She offered no help in Jim's single-handed take-down of the small military group (who have both automatic weapons and military training), except to lead Hannah around and be dragged along by Mitchell. Then she hesitates to kill Jim whom she's uncertain is infected (why didn't Jim just say something here?) and even Jim remarks that it was out of character for her. I suppose it's fair to say that not every character has to have one or two personality aspects that they have to follow and can show various other, radically different sides of themselves. Still, Selena ends up straying a long way from the quiet, silent and focused heroine she was in the first half of the movie.
All in all, I'd say the filmmakers did a bang-up job of making her very emotionally invested in all of these characters. I got nervous as hell when Jim gets shot (and I have read about the alternate endings, which gave me flashbacks of I Am Legend) but I was relieved that he pulled through. I really want there to be more characters like Selena who are beautiful yet strong ... just without any sort of rape aspect tied to them.