Change in discharge and pains?

Bryony • I'm 22 years old soon to be 23 :) and pregnant with my first a little boy. Joseph William Anthony Pratt Due 06/08/15.
Suddenly today out of nowhere my discharge has has a yellow tinge to it and I've been getting rather bad pains that seem to start in my lower bump and shoot down into my vagina area I was wondering if anyone has experience this? 
I have a mw appointment soon so will be asking. Thanks.