I have a theory about the stinging which may or may not be true. I am a nurse who now works in a med spa 2 days a week, my retirement job. Been a nurse 34 years. All I do is poke people all day. IM injections, IV’s, and blood draws.
Some people tolerate it, some don’t even though they are all doing it by choice. Each individual has their own tolerance for pain. One time it hurts the next time it doesn’t. Our reactions are often not correlated to how we think we feel.
My best advice is reconstitute early, don’t inject cold peps, and be quick about it. It shouldn’t be the needle that hurts, and occasional burning without a site reaction might happen without a reason.
PS I am not discounting anyone’s personal experience and I’m not saying your doing anything wrong.