Both of my parents got the 1st vaccine dose on 2/2. Yesterday, they both felt bad. My dad was just tired. My mom had low-grade fever, aches, no loss of taste/smell. She was convinced it was the flu so she went to urgent care. The doctor did a flu and COVID test. Flu was negative, COVID was positive. She asked the doc if the vaccine could cause a positive test. Doc would not really answer.

My mom feels better today. She is just tired. My dad feels fine. They plan on staying home for 10 days just to be safe

So, my questions are:

Is this a delayed reaction to the vaccine? Can the vaccine itself cause a positive test with or without symptoms? Is it really COVID and she will have a mild case because she had the first dose?

To be clear, I don't think she got COVID from the vaccine. I am wondering if this is an immune response and/or if the vaccine can cause a positive COVID test?