I have two stress fractures on my knee they are opposite of each other and I have no idea how it happened. Is this bad for me and is this uncommon?

Not commone in knee. Most stress fractures happen in the metatarsal bones of your foot with repetitive activity such as running or marching in the military while you can get them in other weightbearing bones in the legs they are not very common. You may get some blood tests to check on your underlying bone metabolism or even a bone density fortunately most will heal fine with rest.