Ovulation bleeding? What is going on!?

Sarah
Hi ladies, 
I need your help. I am on my 4th cycle of trying to convince and just started taking opk tests 3 days ago. I am on day 17 of a ~35 day cycle. This morning's opk (and the prior 2) said low fertility. However, this afternoon, I am experiencing lightish spotting. 
I am at a loss as to what this is and frustrated that since going off the pill in July I don't seem to understand my own body anymore!
Does anyone have ideas as to what this is? My husband and I have been having sex about every other day. 
Thanks in advance for any thoughts you may have!