Painful injection plus worried about 5mg

I have a couple of questions, I'm being a bit of a worrypot today :( I did my first 5mg shot today, but for the first time the injection was really quite painful (for reference I did it in my thigh). They haven't been up to this point and I have a fairly high pain tolerance, so I'm a bit worried. It's already bruising after a couple of hours and I don't bruise easily at all. I know the needle isn't long enough to cause any damage, I'm just curious as to why it might have happened.

On another note, I've seen loads of scary things about 5mg causing horrible side effects that stop people taking mj altogether! I know it's not reflective because people tend to only post something if it's especially bad but I just need some people's experience of 5mg so I don't freak myself out!