Misc I really don't get how iv allergy/sensitivity works

Sometimes I can miss a whole shot and it won't even hurt but just swells and turns red. Then with the exact same stuff at a different time I can get a direct hit leaving almost nothing out of the vein, only to have my whole arm immediately burn itch and have pain upon injection followed by intense patches of hives and redness.

What exactly is your question?

Why does sensitivity happen at some times and not others all else being equal?