Be proud. Some people in my class can't even get a C or scrape a C despite being in top set.
Once you learn the entire GCSE syllabus for your subject then you will find it a breeze, especially with science! Also, if you're worried about not doing Triple Science, speak to your teacher or your Head of Faculty and I'm sure they will sit you down and explain whether or not you'll do it. Either way, with that grade you're doing Triple Science I would think.