I received a letter from a cancer ad preventive center could it be cause they found cancer? Thanks

Not necessarily. It is best to read the letter and find out what it is about. Many cancer centers also run cancer screening and cancer prevention programs for which they often recruit regular folks who have no cancer. just read the letter carefully and then react appropriately.
Need more info. Did you have a biopsy performed recently or any blood tests performed? Your medical reports/lab tests are all private/confidential under HIPAA. By no means would anyone else besides your provider (physician) ever notify you regarding a cancer diagnosis if you indeed had a biopsy recently. There is no way for anyone besides your physicians' to know your medical history unless you release records.