i was practicing on my electric guitar and i wanted to find something on my phone. so while resting my arm on the strings of the guitar i placed my thumb on the home button (i have an iphone 7, which uses the fingerprint sensor home button which is a sapphire crystal and uses a complimentary metal oxide semiconductor capacitive touch sensor to detect the fingerprints electrical charge or something right?  ) and it gave my arm a very small electric shock! the amp also gives feedback whenever i place my thumb on the home button as well 

however, it seems this only happens when my phone is charging! 

 i think i might have some idea of what’s going on here... but i’m not too confident so if anyone could suggest why this is happening that would be great! thank you

Incorrect or lack of grounding can result in the chassis to build up a charge along with everything connected to it(including the strings). Button works via running a small current through your skin so touching it is likely completing a circuit for the charge to flow out while the phone is charging.

I'd look at what type of plug the guitar has and possibly check that the wiring for the building is correct. Can be a safety risk. I recommend seeking someone local to ask about it when you are able.