Positive Negative Positive?

On CD 12 Am and PM I had a positive ovulation test followed by days of negatives. Now on cycle day 16 I have had yet another positive. I am lost? I tested again due to the fact my cycles have been 39 days the past 2 months opposed my typical 31 day cycle and the 12 seemed too soon IMO. Anyhow I heard that your body can gear up for ovulation but you do not ovulate on the first go. This seems to have happened to me the past 3 months and I am at a loss. Anyone experience anything similar?