I sprained my ankle 3 days ago and began bearing full weight on it today. Now its back swollen and is starting to bruise. Did I start activity early?

Possibly. First I would just make sure you have been seen and had an xrays just to make sure you are not walking around with a fracture. If xrays negative, then yes it is not uncommon for people to try to increase their activities and if the pain and swelling worsen to conclude they did too much. You could just rest it more and then more gradually increase your activity when you are doing better.