Can I contact the referee if I know their identity after a double-blind reviewing process?

If the review is unclear, you should contact the editor for clarification, and not the referee. This would be true even if the peer review was not blinded. The editor is in charge of peer review.

No you should not contact the suspected referee. You may think you're 100% sure but there is still a chance you're wrong. Also anonymous reviewing is there to give the referee a chance to be honest and critical and thus not fear retaliation if they reject a paper. You emailing them (if you have the right person) breaks this. Would you be as critical a referee if you started getting emails from authors after rejecting their papers? Especially if the author is more senior than the referee?

If you want more information, ask the editor. You can also ask the editor to ask the referee if they wish to be de-anonymised to make communication easier. Also at the end of the day, the editor rejected your paper, while the referee only suggested rejection. This may seem a minor distinction but it means it is the editor's decision (who could if they want reject the referee's suggestion if they thought the referee was wrong).

If you sent me the email, here is how I would reply:

Dear High GPA

I am not in the habit of telling people if I refereed their paper - that would be a most inappropriate thing for me to do, and it is similarly inappropriate for you to ask me to break the referee’s anonymity. So I cannot addresss any of your questions. If you need feedback about your paper, I suggest that you talk to your adviser or a colleague.

Sincerely etc