Weird Spirit flight number NKS245A

Hi, I was wondering About a flight Spirit Airlines did that Just landed in Portland not too long ago from San Diego, The flight number was NKS245A or Spirit Airlines flight 245A. I was just curious about this flight was wondering if what’s a charter or something. It’s just I’ve never seen the letter “A” after the flight number before.
flightaware.com/live/flight/NKS245A