Weird vibration on my phone only when I touch it..

So I have a (5s) and im having this really weird problem. When I plug my phone into a charger (tried several different ones) and I run my fingers across the back I get this kinda smooth vibration, but it only happens when I touch it. If someone else tries to do it doesn't make a sound or anything unless I'm touching the phone at the sametime. I also get fatigued and bad thumb shakes Wen I hold my phone in one hand for to long. I don't know if im passing a strong current through my phone or what.