My elbow hurts when i fully stretch. what did i do?

It depends: If the pain is medial and goes into your ring and little finger, it's probably cubital tunnel syndrome, which is a pinched nerve on the inside of the elbow. If its the outside of the elbow, it may be tennis elbow syndrome. Both should be treated with rest and anti-inflammatory medication, but if it keeps hurting see a doctor.

Elbow pain: It is unlikely that you injured yourself by simply extending your elbow. There are a number of common elbow problems that may hurt when the elbow extends. If symptoms persist or recur, consult with upper extremity surgeon.