Hey, thanks for your question! Nothing to be worried about! I had a student that scored a 486 on the diagnostic and ended up with a 516 on the real exam. The diagnostic is not meant to tell you where you will end up, but just where you are starting. Most people take it having done no mcat studying and as a result score lower than their expectations. I would just see it as a starting point to measure progress against as you study. The diagnostic has no bearing on what your final score could be as long as you study hard and follow a schedule to complete content review and do ample practice.
I hope that helps and if you have any questions at all, please don't hesitate to reach out!