Well, your best bet is to call Verizojn and tell them your internet is a lot slower than it used to be.

My point about speedtests can be demonstrated easily, I made a speedtest earlier after reading your post, it told me I got a 20MBit connection and my max download would be 24xx.xxKB/s, now I downloaded a demo from a fast server and got this:


Which is about twice as much as the speedtest said would be my maximum (I should have up to 50MBit but I'm not complaining ).

If your internet slowed to a crawl when it shouldn't and you checked your PC for viruses and Malware, as I said, contact your provider, tell them about your problem, that it's new etc.
Be nice, be sure that you're right, you may have to insist a bit if they don't believe you (this is why it helps to check your stuff at home first so you can be sure it's not on your side).
IF they don't do anything, claim that you're not getting what you pay for (never had to go there myself although last time I just changed the provider without even complaining), if that doesn't help, find a better ISP if possible.

That's what I would do but AFAIK in the USA the ISPs are few per area unlike here in Germany where one gets a few to choose from.