bottom leaves are yellow!

hi friends... im worried. my plant 2 days ago was vibrant and green and all of a sudden the bottom fan leaves have turned totally yellow. not all of them but a lot. I thought i read somewhere that that is normal after the 4th week of flowering. and thats exactly where im at.. should i be worried?

its a N defencieny..add more nitrogen for the next few days to a week an you'll see it pick up...but yes slight yellowing is normal...