So I just went to get it checked out by my regular GP (since the other doctor said to keep an eye on it and it didn't fade) and he says that it's a big dark bruise because the mosquito hit or hurt a blood vessel, which they normally don't. It takes a while to fade away he says and apparently it doesn't hurt because you don't hurt any tissue like you do when you bump into something. So thankfully no tick bite or Lyme. But thanks for all your replies, I appreciate it!