Aching behind knees at night. Have a superficial varicose vein. Could that be causing it? Worried it may be PAD, and I'm about to start oral BC.

See Vein Specialist. It is very unlikely that you would have PAD at 25 years old. The varicose vein may be a source of your problem but because you have a vein does not mean that it is causing problems. See a vein specialist for a venous evaluation before starting OBC pills.