reading an interview with her or something when The Time of Angels/Flesh and Stone came out, and she did say something to the effect of not wanting to flirt as much with Matt Smith because he was so much younger than her and she didn't want River to come off as a cougar. I don't think his Doctor seems that young when he's in character anyway, but it's understandable.

In a more meta/continuity approach, the River we've been seeing this season is apparently much younger than the River we met in the Library, so perhaps her different attitude can be attributed to that she's in a different place in her life.