Try disabling the Network Manager's dnsmasq plugin. Read here.
Let us know how it goes.
I'm able to get back online. I don't know what fixed it. After trying papibe's advice, nothing worked and I called it a night. I connected the Ethernet cable to my desktop this afternoon and I had a slow connection. Google's homepage showed up, but the logo couldn't load. I figured that one of the changes I had made the night before was responsible for the slow connection (and for the fact that I was online to begin with). I uncommented the line "#dns=dnsmasq" in NetworkManager.conf and ran sudo service network-manager restart and now my connection is back at 100%.
I would like to thank papibe for his assistance. I'm going to reboot to make sure I haven't celebrated too soon.