Sometimes, it is an awkward moment when you lose your balance, but the pain quickly fades away and you go on your way. But the sprain could be more severe; your ankle might swell and it might hurt too much to stand on it. If it's a severe sprain, you might have felt a "pop" when the injury happened.
Maybe you came down too hard when you jumped to catch a ball. Perhaps you stepped into a hole and twisted your foot. You might be the newest member of a common club: people with an ankle sprain .