Mickey, the soreness you're talking about is normal. It's usually just a slight soreness around the area of injection the following morning. If you hit a nerve, you will know immediately (feels like a very pronounced sharp pain when you're inside the muscle), and it can cause some extra soreness around the site the next day. You could have done something wrong to cause so much soreness but it will go away in a couple of days.
Go to SpotInjections.com, they have lots of useful info and it will definitely help you out. Just stick to learning about glute/quad/delt injects, any other injection site has much more room for problems.
What size needles are you using (length/gauge)?
I used to do glutes all the time. Lots of people prefer glutes over any other body part but once I did the thigh, I loved it. I can aspirate easily, doesn't hurt whatsoever, and I can do the whole process easily, whereas the glute its more difficult. It's entirely up to you, I think you should try both and see which suits you best.
Good luck!!