I had a dream about getting good exam results and it worries me!?

I took my GCSEs in may and june and ever since i have been worrying because my target is at least a C to get into sixth form. In all my exams I feel like I could have done better even though I tried my best and I keep wanting to study and improve even though the exams are over its weird. And I have this dream book which tells you what your dreams mean and stuff and often when you dream something good it normally means something bad, its confusing. So when I had this dream about getting really good grades it worried me because it could mean something bad like maybe not passing!? Does anyone know what dreaming about good grades means?