I was just diagnosed with a sprained ankle(right foot) now i have large purple blotches on my right foot on both sides under the ankle.whats wrong?

Purple spots: Most likely purpura or mild bleeding under skin from the sprain. Should go away with cold compresses, elevation and time.

See below: Could be bruising from the ankle sprain. If concerned have the doctor look at it.

X-ray : I hope an x-ray was taken and doctors aren't utilizing that dumb ottawa rule. You would not believe how many ankle fractures or anterior calcaneal fractures i get bc the er doesn't take an x-ray after a sprain.

It is bruising: The purple blotching you describe sounds like bruising. You may have [just] sprained your ankle, but where pain, swelling and bruising is present, you want to make sure you do not have a more serious injury such as a dislocation or fracture. The way to do this is with x-rays.