PR Update?

Just this morning I notice one of my site has drop its PR from PR3 to PR0. I checked some other website site and found out that other lose 1 PR and some gained 1 PR. The last PR update I remember was on January 2008 and PageRank update happens every 3-4 month starting from the last it was updated. So if PR was last updated on January the next PR update should be in April or May but when I look at my Google toolbar it went into PR nothing “0″. Just to make sure my Google toolbar is not glitching I used a PR checker and the funny thing is it indicates that my website is still PR3. I’ll be observing this on the coming day