Thats a common complain for when someone first opens their third eye. Try resting and doing some healing after and/or before you do a third eye practice. Always make sure you do meditation before exercises. And I find the amethyst helps me with this, as it helps with headaches and strengthens the third eye at the same time.
If you are getting "3rd eye crunches" it could be because you have a block in your 3rd eye or even in one of your other chakras. As all your chakra are connected and have expressive and receptive qualities. About 10yrs ago I became a polarity practitioner and participated in a program called RYSE developed by Nancy Risley. This is a great program for anyone,it teaches you how to open clear balance and repair your energtic systems.I dont know if this is avalible to you. In the meantime you can do a very simple clearing on yourself by focusing on the individual chakras, simply surrond yourself in white light allowing it to flow down thru your crown chakra to each one below then bring the light back up from the base,go back to your 3rd eye and back to your heart charkra complete by allow the light to flow back down and out your base. Ask your higher guidence for help if need be. You may also hold a clear quartz crystal. If you need color add indigo to 3rd eye. Hope this helps