Yeah, EpiPens are easy to use but under stress you can do anything simple wrong. During training we were particularly told to avoid the thumb thing - it can be quite serious as you lose blood flow to that area from vasoconstriction from the adrenaline. I hope yours is OK but if it feels numb or you can’t move it properly you might need medical attention.
EpiPens are very much not the first line treatment for hypoglycaemia (low blood sugar; generally occurs in type 1 diabetics from not eating enough or having taken too much insulin) but they can work in the short term and they’re usually cheaper than injected glucagon, which is the first line (along with giving the person sugar - jelly sweets, buccal sugar syrup, etc). Generally you hope to head off the episode before it needs medication with sugar.