This is a common issue right now, from what I hear--ISPs putting a tight cap on Internet users who go overboard on their montly bandwidth usage. Don't know how much you'll be able to do about it (they probably redefine "unlimited usage" somewhere in their legalese), but you're certainly not the only one facing this kind of problem.Still, though... 60 GB in one month?! That's bigger than my hard drive.
I used to work for a satellite internet company (not gonna say which one, but they sucked) and that particular company called it a "Fair Access Policy" (lol FAP), their limits were a LOT LOWER because of how satellite works, but all ISPs have that cap in case someone in the area is stealing everyone else's bandwidth. It's pretty high for cable Internet, i think the OP said like 60Gigs, but it's still there. Normally your speeds go back up after a month-ish.