I know with me I tend to isolate when I feel over stimulated with my sensitivities. I know that I do this when I need to regroup and spend my mind's energy on collecting data by either reading or using visual entertainment. I would think that this burnout happens to most Aspies. It may be that your autism is not getting worse but your tolerance to the outside world may be declining as you are most probably pushing yourself rather a bit too hard and you may just be stressed. So I believe yes this phenomenon is valid. It happened to me when I was 36 years old. I just crashed and burned. I just needed a break. I don't think it meant that I was going down the spectrum, it just meant that my tolerance to the outside world was getting of shorter duration.