You just came off of BC, you could have O'd or you could have had an annovulary cycle.
No one here will be able to tell you what happened.
All you can do is continue on to the next cycle. Temp every day (even if you're sick and being busy isn't an excuse unless that means that you're going for several days without sleep. If that's the case, then you have bigger issues here). Continue to do OPK's twice a day and check CM.
It can take several months for your body to readjust, so if this was a wonky cycle, then you can just assume it's due to that and move on.
GL with this next cycle!