Within 24 hours, I noticed that my download speed had dropped considerably... to the point that some sites were timing out. I thought it might be too much traffic at the time. So I waited 24 hours.
Same problem.
So I checked my BigPong a/c, and found that I had exceeded my 8GB limit... to 11.6GB!

(I normally have difficulty exceeding 2GB of my allowance.)
BigPong had slowed my access speed to dial-up speed! AND I'll have to pay through the nose for the excess GBs. Double jeopardy!
I checked the only s/w I had installed - BitTorrent - and discovered in the Preferences that it automatically connects when I boot up, and allows others to upload the (only) movie I have on my hard drive! So I'd been feeding the movie to multiple people for 48 hours...
Obviously, I changed that preference.
So if you have a plan with a download limit (which should be called a 'Traffic Limit' because it includes uploads as well) be aware of this setting in BitTorrent.
Same probably goes for all such file-sharing s/w... check the Preferences when you install.