Positive OPK IMMEDIATELY after period

Hi Ladies!!

So just wondering if it’s possible to have a positive opk right after a period ends? My cycles haven’t been regular lately- their usually 31 days; however my last cycle was 21 days and only lasted 4 days (if that) and ended on the 30th. I Decided to test earlier as I feel as though I’m missing my window. Has this happened to anyone, is this possible? My husband and I BD just Incase. This is the highest positive I’ve ever received.

Any advice would be great- thanks!