I've experienced similar things... with like 20 in unarmed and some good'ole brass knuckles heads were exploding. pretty sure it has to do with the high dmg output and the new DT system they implemented.... seems as though melee weapons and unarmed ones do more hard dmg than guns unless your guns skills are up to par. you can slug 15 bullets into a guy but if you have low dmg compared to his DT the bullets just bounce off'em.
They went overboard with melee in this game. My first character focused on guns 100 and I was about 60 hours in and even with 20ish unarmed skill with brass knuckless I was keeping up with kills versus my maxed out guns.
Right now my second character is level 11 and is focusing on melee/unarmed/survival while using a baseball bat one shotting everything that moves. When my guns character was at this level I was struggling with shooting some enemies with two clips of bullets just to bring them down.
Believe it or not, my guns character used way more stim packs and food compared to my new character. Kind of counter intuitive because I'm running in head first on this guy where as the other one I was getting behind cover for reloads etc.