Why does my internet randomly slow down?

I have a portable Wi-Fi modem which plugs into the computer via a usb charger that randonmly slows down for no reason. It only has four gigs so i don't download anything or watch any videos all i use it for are facebook, skype, minecraft and instagram any idea why it does this? And its not because of a blocked signal because i never move the modem from its spot so it can't be a blocked signal.