How Can My Fully Vaccinated Fiancée Test Positive For COVID After Multiple Negative Tests, And Should We Follow Positive Test Guidelines?

My Fiance got both of her COVID shots 2 months ago, which by the CDC standards grants her immunity to the virus. 3 days ago she had to get tested for work as she usually does and the first test came back positive and then they made her take 4 extra tests after that which all came back negative. She then proceeded to get a test at a Doctors office and it came back today displaying positive. This should not be possible if the CDC and Government has come out with a vaccine which they claim works. How is this possible? and should we follow the guidelines for a positive corvid test?