Invitation to book for smear


I have a little bit of history with regards to bad smears and am subsequently on yearly checks.

I had my letter today, dated 14th February 2018 inviting me to book with my GP for a smear. I called them to make an appointment and they said its not due until 19th April and that if they were to do it, it wouldnt be accepted. Why have i had the letter if this is the case? Has anyone experienced this before?