I just wanted to pop in and say that I hope you took Rich's advice before working out again. Your doc can determine what's causing your headaches and give you the best advice on how to proceed with your exercise routine.
While you're waiting, here is some information that may be helpful:
If you've never tried yoga for fitness, stress relief or just a little Zen in your life, this is a great time to stretch your exercise limits! (Just remember, as with all exercise it's best to chat with your doc before jumping in.)
Loved yoga in the past but gotten out of the habit? Begin anew.