I've been doing IM injections for over two years and yes, it feels like a bruise for a few days after each injection.
There have been times I've hit a nerve and it is like an electric jolt. I've also hit veins and become a bit of a bloody mess.
I've injected into my thigh but have never done a buttock injection. I'll try it one of these days, but I haven't so far.
-Sandy