Changing ovulation days?

Alison • Finally pregnant after 5-1/2 years and losing my mother on 11/1/18. I got pregnant the week she died and just before my husband got a vasectomy. I never thought it would happen for me. Life sure is crazy!
Does anyone else experience this? 
The first time I checked my ovulation day I had a surge on day 17. Now, 3 months later, the last 2 months has been day 11. Is it the ovulation kit brand or can your ovulation day actually fluctuate?