You may want to contact your ISP and find out if they are having latency issues in your area. If the signal has to routed more to reach your computer the ping will be higher. Any changes on your system at all?
Has nothing to do with your modem. When your ISP has heavy latency on their servers, they have to route and reroute the signal more to get it to your computer. This causes heavier bandwidth consumption on their network which consequently increases the ping of your connection. You'll notice slower browsing, lag on servers, etc.
Dunno why dont you give it a week then switch ISPs?
Run a trace to find out where the problem is? I heard of someone on here having a higher ping every time their dog sat on the phone wire next to the computer lmao
Its not unusual for your ISP to go tits up once every few months, but not normally for longer than 2 days.
Make sure in the Multiplayer options section you have it set to cable and not something else. This seems minor but I had this problem before after a reinstall and forgot to change it. I was getting all kinds of weird ping spikes. The default setting is ISDN. Other than that it would have to be your cable.