There were two versions of River at Lake Silencio in "The Impossible Astronaut". The River in the spacesuit didn't remember the events because they hadn't happened to her yet. The River who was watching with Amy & Rory did remember but lied about it.
As River explained to Amy at the end of "The Wedding of River Song", after the whole shooting thing had been sorted out, both the Doctor & River lie often to avoid giving "spoilers" (foreknowledge of future events).
In "The Impossible Astronaut", River held Amy and Rory back until after the final shot that seemingly killed the Doctor, hinting that she knew what was happening and had to let it happen.