This sounds very familiar to when I was a young man and I was given a lot of painkillers and had a very high fever due to an infection. I had a very intense waking dream where I felt like I was hurtling into an infinite void. I had a few flashbacks in the next week while totally sober. Overall it was an intensely unpleasant experience.
When I was about 11 I had a bad flu with fever for a number of days. I was eventually hospitalized for dehydration as I could not drink or eat without vomiting. I don't remember a lot of detail but there were several periods of vertigo and a blurring of dreaming and being awake.
