I am 19 years old and weigh 125lbs at 5'4. It is hard for me to gain weight. I recently went from weight lifting 4x a week and doing cardio 2 or 3x a week to now weght lifting 6x a week and 1 day of cardio. Is this OK? I do not want to get too much fat as a result of the diet I am on. Should I do cardio another day as well since it doesn't burn muscle?

Have you any idea what your bodyfat is? If it's very low, then you risk burning muscle with cardio, but far more people worry about this than actually are at risk. Most people can belt away with no danger.

However weight training can be cardio too. Ever taken your pulse after a set of heavy squats?

What are you eating?

I eat lots of meat and protein and carby things. I have a hard time gaining weight and I have a high metabolism so I can sort of get away with that than most people can. I have never been fat before. I went to many sites and on average, by body fat percentage is 10-11%