I can't stop thinking the worst..

I think you are reacting quite normally. I've  finished chemo and still experiencing side effects but secretly worry they really are metastases not picked up, not side effects, sigh ..... just keep pushing those thoughts away. I believe eventually we will start to relax a bit ...but it will take time. Following up on issues isn't  a bad thing...but I hear you. March wasn't  long ago. It hasn't  been even a year since diagnosis so I imagine you won't  feel confident for sometime yet.

If it becomes a preoccupation in thinking then counseling around it might be an option.