I've been having really bad dreams the past several nights and don't know

what's causing it. Any ideas? I've had dreams lately where I wake myself up because I'm yelling or crying. Once I even kicked something in my dream and woke myself up because I actually kicked one of my cats off the bed. Then of course the dream was so bad that I can't go back to sleep. Why is this happening? I'm not doing anything different before going to bed so that can't be it.