5th Knee injury?

Ok this is my background, all on right knee!!

3 grade 2 ACL tears
1 full ACL rupture - reconstruction surgery
1 large meniscus tear, section removed.

All in the sapce of 9 years with the last injury happeing October 2009 and operation November 2009.

Back in my kickboxing class last night and my knee 'popped out' twice on me while sparring. No major pain but definately pain. Was able to walk, jump, bend normally after about 10 mins and feels ok now.

Any ideas what i may have done?