My hamster will just start squeaking really loud out of nowhere.

Hi I have owned dwarf hamsters and syrian hamsters and on times all of ours would squeak out on times, really worried me and like you I'd heard that it was a sign of pain or stress. However nothing had been changed or altered with ours either. I think sometimes they do it in their sleep, maybe they can dream like we do and perhaps in the dream they were annoyed with something. I wouldn't be overly concerned if they seem happy and healthy.