My phases have been progressive rather than alternating.
Throughout youth and early adolescence, my greatest fear when going to the doctor was the inevitable buttock injections (penicillin when ill and booster shots when I wasn't) - especially the humiliation when I had to bend over the lap of a receptionist or p/a for her to watch me get them in my vulnerable bare bottom. Further embarrassment was betraying to others where I had been injected if I couldn't sit down later because my friends all regarded getting shots in the arm as a rite of passage. At the same time I developed a fascination about the ordeal, enjoying hearing about others (particularly girls) getting shots back there and even fantasising about getting them myself so long as none were imminent.
My adolescent girlfriend and I indulged a mutual fetish by giving each other simulated injections in the bottom when playing doctor, even though we both remained afraid of the real thing.
Things came full circle in late adolescence when a misunderstanding with a beautiful nurse led to her insist on giving me in the bottom a shot I could have had in the arm. Humiliated though I felt, I was by then mature enough to appreciate the sensuality of a pretty girl baring, swabbing and even injecting my erogenous naked cheek. Ever since then I have taken all shots back there and enjoy getting them like that, especially from attractive nurses. I still experience nervous apprehension when waiting to get them but it now turns me on.