How do I know if I have OCD, or if its depression, anxiety or overthinking?

[LouieSax]

Basically, as the title says above, it may sound strange but I'm honestly not sure if I have OCD or whether I'm just anxious, depressed or even if I'm just overthinking things. I know I should go to a GP if I have any concerns, but what if I am just overthinking? The reason I'm concerned is I always have this horrible feeling inside me, which I interprete as anxiety but then I also have quite dark and depressive thoughts (so maybe that's just depression? I'm not sure) The reason I'm curious as to whether its OCD is because since my GCSEs in secondary school, when I have exams on or even coming up in the next couple of months, it makes me anxious and for months on end I will spend all day everyday doing work and checking my work to make sure its right, even when I've written my assignments I will continue to check them constantly, taking up my whole day of just checking my assignments, the rest of my life pretty much goes to poop, I stop cleaning my room, I stop going out, I stop doing everything. I'm not sure.