Swimming with flippers gives me a sharp stabbing pain in leg...?

You might need to adjust your form. I know that when you walk and you take too large a stride you can hurt/injure your upper leg and hip. If you have a swim coach let them know the situation. There might be a way to adjust your form a tad so the pain doesn't happen anymore. If the pain is consistant, like it happens more than a few times a week or a swimming session you really should see your doctor to avoid any permanent damage.