What's This Yellow Stuff?!?

I've been noticing yellow stuff in my undies for the past 2 weeks, and I dont know what it is i mean i always change my undies literally everyday but whenever i check my undies even if thats after 10 minutes i put on new ones there will be the yellow stuff. This may sound stupid but i got really worried and smelt my undies when the yellowish stuff was there and it smelt like urine. It isn't that thick but it feels wet for half an hour and after when i check its all dry. I'm pretty sure that i dont have a disease because 3 weeks ago I had a horrible stomach ache while i was shopping with my Mum and we went to the walk- in centre they did a urine test and said 'Well your urine is fine' so yeah. My friend said 'Well you could hve a Discharge but if it was a Discharge it would be white and thick' Is that true if Im havin a Discharge (What is that) does it HAVE to be white and if its yellow is there anything wrong eventhough I doubt it. If this is a sign of my Period then how long will I have my period in? Thanks 10 Points bEST Answer