It changes for me too. I can go from one to another rapidly. Usually only have one or two at a time. More than that would be chaotic as hell, wouldn't it? But yeah I just get very interested in a fixation, study it, make it an expertise. What is so mentally disordered about that??
However I do recognize that the fixations can become problematic--like the one I had with skin care (and the removal of pimples). I ended up trying so many "home remedies" that my face was red and peeling, several times. This was about 5 years ago.
Ones I've had - 1977 punk scene/music, drugs (a big fixation to this day, and not just using them--studying them extensively, reading all about them, for hours and hours)--heroin, meth, cocaine, lsd, hallucinogens of all types (dmt, shrooms, all forms of psychotropic plants), even pharmaceuticals, & pot as well, psychiatry (studying DSM, and the like), psychology, existentialism, school shootings (Eric and Dylan were particularly a fascination for a short period), different music bands, different movies (writing fan fic, obsessively finding out all about them, reading scripts, etc etc), the list drags on......