Knee giving out? Slightly worried.

18 year old, female soccer player. I tore my ACL in april 2011, and underwent ACL reconstruction surgery with a hamstring graft the first week of october 2011. It's been about 7 months since. I've been practicing and all that jazz, and today was my first game in about 20 months in total. I'm in a tournament actually. However by game three, I went to push off with my left leg (the injured one) and run around this girl to get in front of her. My knee gave out. No popping, no pain, no swelling, no lack of function. Just gave out. It scared the crap out of me though, so I got off the field.

Is this normal? Like, I feel as if it's not. I'm not sure if I should play tomorrow, or anything. :/