In addition to the responses of others here, make sure something else is not going on, healthwise.
My sister was out to visit us here in Colorado, we're at ~5400 feet. They flew in to Denver (from Alabama - elevation about 600 feet!) and she commented about feeling a difference immediately upon deplaning. We went to a restaurant for lunch and she was very pale and weak, and passed out in the lobby. After lots of hydrating, she got to feeling a little better, but not back to normal. My brother-in-law took her to an urgent care and they told them to immediately go to the emergency room at the local hospital. They did, and the ER staff put her on oxygen and ran some tests. Turned out, she had the flu and bronchitis, which were exacerbated by the change in altitude. She had no idea she had the flu, didn't have the "typical" symptoms. Given a couple of prescriptions and told to keep well hydrated, and she should get to feeling better. She did, after a day or so.
If you have any other health issues, they could be contributory to that altitude sickness feeling. Even those of us who've been here a long time at altitude can be affected. The multiple times I've been to the top of Mount Evans at 14000 + feet, I've definitely felt it and had to get down to a more comfortable 8000 feet to feel better.