It really hurts below my ankle, what could it be?!

Since it is the morning after, ice won't help. Warm compresses are better, to increase circulation and promote healing. Ice should always be applied immediately after an injury to reduce swelling and edema and to calm things down. But if applied more than an hour after an injury it is useless.

You probably need to get a different running shoe. You likely came down hard on it at an angle by stumbling or turning suddenly as you were landing on it. Elevate it, massage it, warm it, take aspirin or tylenol for the pain. Personally I like aspirin better because it reduces pain, and thins the blood a little bit so it circulates around a wound better. Just don't take more than the bottle says. Sometimes soaking an injured heel/tendon area in a pan of really warm salt water will help it to heal quicker. Try not to aggravate it by running on it or walking on it excessively for a few days. It needs to rest to recover. It will still be sore tomorrow, but by Monday it will feel better...but don't go running until the pain has been gone at least two days. Then work it back in.

you probably are starting to get some tendonitis on that tendon. or sprained your ankle those two ligaments are common to sprain. i would ice elevate and no run for a week. then take it easy when you return. you dont want to overwork tendonitis or a sprain.

You probably just overworked it, probably running on some concrete or something. Elevate it and put ice on it.

try using different shoes!
make sure there soul is not flat!