Weird thing #2 - What's up with Grover?

I was already having a bad last day because it was my absolute last day...I'd gotten kicked out of Yancy. First of all, Grover (my best friend) kept acting all fidgety on the bus. Grover kept trying to say Mrs. Dodds never existed, but I know that is not the truth. He told me he lived somewhere called "Half-blood," but he said don't say it real loud...I don't know what that's all about! Anyway, while we rode the bus, we saw the craziest looking old ladies knitting the biggest socks I'd ever seen - and they were looking right at me!!! Grover made me promise to let him walk me home. He said he was sent to protect me. Protect me from what???