You can do top of back leg, just feel for a fleshy bit. I prefer to do my goat im injections in the neck as I can hold the collar with one hand and I think it's less stingy than the leg muscles! Aim in the triangle at the top of the neck, above the widest point. The bottom of the neck has all the veins etc so avoid that.
Whenever giving an injection, always suck back on the plunger before injecting and you can feel resistance of the vacuum, so you know you're not all the way through the skin, and you will see that you're not drawing blood which would mean you're in a blood vessel and need to move the needle until this dosent happen.
There is almost no muscle behind the shoulder and it would be all too easy to go into the chest, so although this area is perfect for subcutaneous injections, it is dangerous for im.
If unsure, best to get someone to show you, either another goat keeper or the vet.
Hope she feels better soon!