I know for us as soon as we strted ttc and tracking o my cycles went haywire. My first week charting I o'd about 5 days after I assummed I would (thank God I was charting or I would have thought I was pg) sometimes you body wrks it's way up to o and you may have symptoms for quite a few days until your body is really ready.

It could also be (TMI) leftovers iykwim especially if you have been bd more than usual and are used to using condoms or something?