If your strengths are really focused in one area then getting better A-levels than your GCSE's is not uncommon. I got 4A*'s, 5A's, 6B's and 2C's at GCSE then got A*A*A* in maths, further maths and physics at A-level, (plus a B in chemistry fourth full A-level).
I actually got higher in A-level physics than GCSE physics too, an A* at A-level and an A at GCSE, so it can be done. That being said I worked way way harder at A-level, but I enjoyed it so it wasn't that bad at all.