Allison, I haven't been doing P90X but I am doing Chalean Extreme. Honestly, from what I have read and what I have done, I would recommend more weight, less reps because you will NOT bulk up. The only reason to lift light would be if you are pregnant or just getting back into it after time off. We don't have the testosterone to bulk up unless we're taking some kind of supplement. It just isn't possible in women like it is in men. I have got a lot of definition in my arms and legs from 2 months of lifting 2-3 times per week (and a bit of on and off lifting prior to Christmas) and I am lifting as heavy as I can possibly bear (25lb dumbbells in each hand on squats and deadlifts, and overhead presses at 10lbs in each hand because my shoulders are quite weak) and I am really pleased with the results. I am not bulking up at all, and I haven't lost weight, but my clothes are looser and my arms and shoulders look so much better than they used to. My thighs and legs are tighter and getting harder and stronger and the skin on my thighs (which was pretty cellulite-y) is so much smoother. You should ditch the water bottles and lift some really heavy weights!
For proof that you won't turn into the Hulk if you lift heavy, check out this awesome story: http://nerdfitness.com/blog/2011/07/...ng-super-hero/