What Causes Cough?

I have had a cough for over 3 weeks. I have been in and out of my GP's office a few times now. On the first week 3 pin dot sized blood speckles came up in my mucus when i coughed. This has not happened since. However i still have a constant irritation at the bottom of my throat. I was sent to the Medical Assesment Unit at the hospital to get looked over. They did blood tests and a chest x-ray.. everything came back normal and it was put down to a virus. For the last 4 days i have had on and off sick stomachs.. sometimes after i eat sometimes before i eat. Yesterday was the first time i vomited a small bit after eating. I have also been very tired.

I am wondering if there is anything i should be worried about or could this sick stomach be from stress etc.. I am a bit of a worrier and always think the worst.. Stomach cancer.. or lung cancer for my cough etc.. A bit hard not to. Any advice you could give?