Because your knees are joints that work as shocks for your body movements, they are hurting from the amount of weight applied to them because you are getting more active. They will get better with each pound you lose. 10 lbs lost = 100 lbs of pressure removed from your knees and back. Keep eating healthy, but track calories. Keep your intake to 1800 at first, the decrease ultimately to around 1200-1400 calories. This will make the weight melt off.great job on the change!
It is hard to change everything, and it seems slow at first, but you're on the right path. Everyone has to start somewhere.
For the pain now, take Advil, wrap them with ace wrap for support, and you can also put ice packs on them at night/whenever. A knee brace would also work in place of ace wrap, but costs a little more. Also, use icy hot, Ben gay, etc. Make sure you take your vitamins too, they help with muscle and cell repair.