Last few days, when I check my Google PageRank using an online PR checker, my blog ranked as 4th but when I checked it today it gave me a total shock when I found out that it lowered down to PR zero (0). Was I penalized by Google Penguin? And how do I know if I was? I couldn't think of some reasons why they should do that to my blog. Downgrading my PR from 4 to 0 isn't a joke.