Did you win the lottery last night? I think not and you know why? The odds of you winning the lottery are about the same as you having a venous leak. Now you want that I should list all the symptoms of a venous leak that you could easily look up so that you can worry yourself even more? Not a chance. When you win the lottery come back and we'll talk about the venous leak you don't have.
Don't worry about ruling out the worst first... rule out the easy stuff and make your way down the list. Diet, lack of sleep, porn usage, testosterone levels, anxiety, pelvic floor, overworking during PE, etc etc etc.
It's not that you're being irrational. There are simpler ways to go about finding the cause.