This happens when my emotions run to high, and I get this empty feeling. For mew this relates to the way i handle stress so i dont know if its the same for you. I dont really deal with stress i just kinda store it up. Then after a while i begin to feel sick angry and alone. Then once it come to dealing with another stress i kinda explode with lots of emotions (never good ones) afterwards i feel empty and numb. I dont really feel sad but i dont feel anything else. I dont really know how to help you in this since it only happens to me ever once in a while, and when it does i usualy dont care about anything anymore. All i can say is if you feel a hole in yourself after wards be carful what you fill it with.
I've experienced this and I think we've learned how to block out these bad feelings. Medication can do this as well. I think when we've dealt with a lot and no longer want to hurt we can go into a protective mode which allows us to block out our emotions. It does feel weird, and I'm not sure what the answer is. Take care.