Am I allowed to ask for help if I am currently winning the fight? The rules say I can ask for help if I'm losing, but nothing about if I'm winning.

That's because if you're winning, you don't need help, and a true munchkin would never ask for help they don't need. Unless a card says otherwise (and a couple do, for their own weird reasons), you may not ask for help if you are winning a combat.
Related Rule(s)