yeah this is very common. i did the same thing. it's the tensor fasciae latae. now my gluteus medius sticks out very obviously so it's easy to inject and my favorite spot, but at first it was hidden and difficult to find.
i always start with 1cc or less so it wasn't horrible but my IT band was on fire for a couple days. the worst was doing 1cc in my bicep with a slin pin for the first time. about to do 2cc in my shoulder for the first time and kinda nervous. (have done 1cc)
i do 2x EW and rotate shoulders, ventroglute, dorsoglute, vastus lateralis. i like to add a new spot just to increase my know how so i do chest, bi's and tri's with a slin pin. i'm one of the few who doesn't prefer using slin pins to inject IM.