Just to give you some anecdotal evidence (not trying to rub anything at all).
I wanted to study Maths at uni, so I picked up Further Maths in Year 13, and my offer was A*AA. For Maths, C3 came first and was pretty happy with how it went. Then came C4 - I missed out an entire question because I'd forgotten how to find the volume of revolution of something or something like that. Whilst doing the question, it felt so horrible, and I kind of sat there for 5 minutes with my head in my hands. A friend who was sat next to me told me after the exam that he noticed and wanted to poke me or tell me to get on with it. I also missed out on half a question on vectors, but hammered my C3 and in the end got 100 and 84 to get the A* in the end, so ya know.. that might not be a consolation if you've done AQA C3 today (though I did it with Edexcel) and you might be better at C4 anyway.
Also, with Further Maths, I had to sit M1 with FP2 as well as S4 with D1. I did M1 first and that was fine, but when it came to FP2 I genuinely struggled, and did that thing where you flick through questions until you find a question that you can answer - I got to the end of the paper very quickly. I thought it went disastrously and I was expecting 30-40. Luckily I got 52, got around 80s in the other stats units and closer to 100 in the others and got an A overall, so okay, there's some pressure on you but it's by no means impossible.
What I'm trying to say is that you might get more than you might expect, and you can afford one bad exam or even 1-2 bad questions in core maths and it could still be okay. It's annoying when they mix papers up and make things difficult and it feels like your hard work has been wasted, but it's over and you should focus on your other exams and try to do the best you can, otherwise you'll regret it.
Good luck!