8dpo (or so I thought), positive opk today

Hi friends, 
Today I am what I thought was 8dpo. Took an opk this AM for the heck of it and it was a STRONG positive. My cycles vary from 28-35 days generally, occasionally it's been 41. I am so confused. Is it possible to O twice in a month or should I treat this as regular ovulation? Any help would be great. TIA!