google webmaster tools

hey all

i have google webmaster tools and my sitemap is downloaded by google every day

i noticed over the last week the amount of "indexed URLs" went down by 400 or 500. is it normal for this to happen from time to time? i didn't make any changes that i think would cause this

is this normal to happen every now and then? if the trend keeps happening, what should i do?

thanks

If you are install vBSEO since few months so that's normal but if you install vBSEO since long time so you will need to check if your visitors is "decreased" but if the visitors is normal so that's some problems in Google when update your number of pages indexed