My dog is older and is acting strange. Please help D:?

My dog seems restless. She always gets up to move around even when nothing touches or is around her. She pukes multiple times in the same spot. Her poop isnt hard. She has fleas because of one of my cats going outside and when she bites at them she hurts herself and whimpers. She cannot at all get up my front porch stairs and theres only four steps on it. She is eight i believe. Shes a german shephard named sadie and is beautiful. I really want to know why shes been acting this way? its sccaring me. A couple weeks ago my uncle put down his dog lucky of thirteen years. If youve ever heard of mans best friend thats a great example. What do i need to do with her?