It's difficult to tell when you only say 1 fertile day hun, how long does the fertile period last for? Perhaps it's just longer than usual? If you're not 100% sure on the day you O, it might just be that you get a lil fertile CM, then not then do again at your usual O time. Hope that makes sense?
I just had a look at your charts, but there;s no info at all on last cycle's chart so it's impossible to comment on it

Sorry Cat

I'm no help!