Ovulation/cycle question

Okay, my cycle used to start around the 4th and end the 7th with ovulation around the 21st. The past two months ovulation is around the 27th and cycle starting the 8th-12th. Is my ovulation and cycle changing? It is still regular, as far as coming between these dates. If it is changing how to I update glow? Because glow is way off with my cycles and ovulation days now! I have been using a regular calander to mark everything, even with the use of glow. Help please.