Dust mites / yellow phlegm / Tests clear

Hello everyone,
I'm hoping someone can tell me a little bit about what's going on with me.

A year and half ago I went to a friends house with sick kids and I ended up sick too as well. It wasn't a regular cold/flu. It was something that lasted more and I ended up getting it badder than the others.
It turned into a beginning of pneumonia.
I had Xrays done, they saw the pneumonia. After the antibiotics, they told me the pneumonia was gone but I kept coughing clear mucus anyways.
Then, I saw an allergist.
I had a lung tests done. Don't remember the name. But you have to blow in tubes and they measure it on a computer. You have to also go in a phone booth kind of thing, same thing.
They saw nothing wrong with my lungs even though I kept coughing almost every day, 3-4 times a day.
They did allergy tests.
I'm allergic to cats, all trees, dust mites. Dust mites the most.

I was told to change my sheet every week, spray my bed with an anti allergy spray, clean my house, etc. I did, still do today. I will cough. And once a month it's yellow almost like a highlighter. And it's not 100% yellow, it's like spots of yellow phlegm in the clear mucus. I don't have any other symptoms but that cough with phlegm. I tried changing my diet. Eat less carb. Still cough. I drink a lot of water. Still cough. I exercise regularly. Still cough.

1- Can the yellow cough be caused by the dust mites? Because it's worst in the morning after I get out of bed.

2- Why am I still coughing even if all the tests are clear?

I take Singulair and Avamys nasal spray.

It helped tremendously.
Before taking that, I was sneezing 20 times a day and non-stop blowing my nose.
I can now breath, but that cough I developed after the pneumonia is still there.

I'm worried about the yellow phlegm spots too. But if it's normal, I'll just deal with it.

Thank you for reading and looking forward to read your answers,

Anna

Hey did you ever find out what was the problem and did you get it fixed..?? I have the same issues..it seems like when I fast I my mucus was clear....