Something's wrong with my fish?

I have six neon tetras, 3 guppies, and some shrimps. One has a bump on its back and where the tail connects to the body, it is a bit swollen and pale. I'm sure its not neon tetra disease. It might be a bacterial infection. Sometimes the little guy gets aggresive. Also, one of my shrimps disappeared. I had 3, but now, I can only find 2. I'm sure my fish couldn't have eaten the shrimp. What happened to the shrimp and what could be wrong with the neon tetra? Thanks for answering.