I agree that you've entered a plateau; however, I doubt there's has been muscle loss, since exercise builds muscle, not loses it.
I do agree that trying new types of exercise is good. If the body becomes used to the same thing every day, it's no longer a challenge and you won't use the same number of calories. This is where the plateau comes in.
By switching up your exercise, periodically, you will work all the muscles in your body. There's not much you can do on a treadmill, except walk at different speeds/inclines. If your knees are becoming sore from the treadmill, give it a rest and try something different, as suggested by holysnap.
You might want to look into yoga. It's great for weight loss, plus it helps tone all your muscles........
Additionally, you may need to drop your calorie intake, to adjust for the 15 pound loss. As you lose weight, you won't need as many calories, as you did.
Congrats on the 15 pound loss; don't give up.
The way i understand your situation is that eventually you reach a plateau effect when it comes to cardio training and losing weight. and even though you've lost 15 lbs which is great you probably lost 4-6 muscle pounds and 9-11 fat lbs. and when you lose muscle your metabolic rate slows which explains the plateau effect. to my knowledge there are two ways to counter this effect. 1. is to eat higher nutritional foods and overall eat less calories or 2. start to do light strength training. i believe there are some statistics flying around saying every 10lb of muscle you gain your body effectively increases it's metabolic rate by 500calories a day. My suggestion would be to try something new besides the treadmill like a high intensity dance class to kind of get new body parts involved. and just by working more body parts you could easily grow a pound of muscle on your legs, arms, chest, back and abs.... which will ultimately increase your metabolic rate.
I am a 22 yr old college student with NO health background... i've just read stuff from different sources and got a basic understanding... my point is some of this could be wrong so anyone feel free to correct me.