Take altitude sickness seriously; the advanced stages of it can kill you. Even the early stages are a pain, as you have discovered. I have had troubles with it for years, usually cope with the "climb high, sleep low" method. For me, this can be hiking at 10K+ feet and going back down the hill to a camp at 7K feet in California's Eastern Sierra. The problem for me does seem to be getting worse as I age.
Reputable info can be found online at the U.S. Centers for Disease Control:
http://wwwnc.cdc.gov/travel/yellowbook/2014/chapter-2-the-pre-travel-consultation/altitude-illnessU.S. National Library of Medicine/National Institutes of Health:
http://www.nlm.nih.gov/medlineplus/ency/article/000133.htmGood luck!