What to do if a wasp stings me on my ankle & it swells up & turns dark red?

Elevate. It is common to have such a reaction as part of a sting. It does not necessarily mean an infection or a severe allergy, as local reactions can look impressive.If it is recent, applying ice can help reduce swelling. Elevating the leg above the heart also helps with swelling.If this is happening more than 2-3 days, then perhaps an infection is present.More common with yellowjackets than wasps.
See a DR. You can try to use ice on the area to bring down the initial swelling. It sounds like you may have a blood blister formed. Use antiseptic to area. Best thing is to see a doctor.