Up until a few days ago, I have been able to play games online (through Steam) very well. But this has changed drastically. Now I can't play any online game on Steam anymore without getting extreme ping spikes and disconnections from Steam. (900+ ping on TF2, 1700+ ping in L4D2 for example)

I haven't changed any setting or installed new programs on my PC. And both games worked fine before. I'm on a cable connection.
What could have happened to cause this? Is my internet crapping out, has Steam changed something?

I sadly can't test online game performance with non-steam games because I don't have any at the moment.