well i don't think you need to 'unlock your aura' as we all have an aura, situations, health, and emotionsb can effect the colour and size but its still there. i have never heard of a meditation technique associated with seeing auras, the ones i have heard deal with eyes, where to look, and so on.
regarding your pain, could be trying to hard, could be a problem with your chakra. your third eye is on your forehead, and deals with insight and clairvoyance. if you have it too open, you can suffer extremely painful migraines. on the other hand, if it is closed and you were attempting to see auras [which would require clairvoyance] then it might of been your subconscious trying to force it open, or energy trying the flow through, and it caused your energy flow to crash for a moment.