Hi, My 13 Years Old Daughter Has A White Spot

Hi, My 13 years old daughter has a white spot on the back of her throat. It's the size of penny. It started 2 days ago. She said that her throat kinda itchy and sore. Now she is coughing little bit. She also feels that her chest is tight and she can't inhale fully ( coz when she does, she feels pain in her chest). She tries to cough to get rid of the tightness and she wheeze little bit. I'm suspecting she has asthma cough? . FYI, She doesn't have fever at all. My question is , does she experiencing a flu or cold?. Is the asthma related to the cold?. Can I give her Benadryl before she goes to bed?. Is there any herbal remedies I can give her?