Cycle length changing

Shelley β€’ πŸ‘¨πŸ»πŸ€°πŸ»πŸŒˆπŸ‘§πŸ»πŸ‘ΌπŸ»πŸ‘ΌπŸ»πŸΆπŸΆπŸΆπŸ± Currently pregnant with my rainbow baby boy, after a loss at 10 weeks 2 days and a second loss at 17 weeks.

I went from a 33 day cycle in November to a 28 day cycle in December. When should I start using opk’s? I’m currently on cycle day 10.