thrush or white tongue from allergies

Since may I was experiencing some extreme coughing with lots of green mucous.  In august While looking at my tongue I notcied it was very white.  I can scrape the white stuff off but it soon gets worse throughout the day.  If i dont brush it, it is covered.  Towars the back it appears it each individual papillae is distinctly covered.  i also get a real bad dry mouth, horrible breathe and often dry lips.  

Im not sure if I have always had a white tongue but I dont rcall it being like this.  i went to the doctor and was perscribed some allergy/sinus medicine.  Since then, the cough and mucous is pretty much gone, but the white tongue is still there.  i thought i had post-nasal drip.

I am scared its oral thrush.  I did have my doctor take a sample of it and he said there was no indication of thrush, however I did brush my tongue farely well before going in.  
1.) Is it possible i threw off the test by brushing my tongue?
2.) does this sound like thrush?  
3.) Can i pick up sever allergies such as this without never having them before?

I understand its hard to diagnose with looking at it but any help would be great.  Thank you

[B623]

any help please