Whenever I sing..?

I ALWAYS get.. mucus or phlegm in my throat 1/4th, 1/2 or 3/4's way into the song, and it's really frustrating and annoying when I have to go 'AHEM' or *coughh* in the middle of a song just so my throat can be clear again. I've tried drinking water and all that, and it still happens. Anyone got this problem and/or know what to do? :[