False positive?

I *think* I ovulated 3 weeks ago with last period 3-4 weeks ago, I'm using natural cycles to prevent pregnancy as I've got a 1 year old. My BBT hasn't come down and my period hasn't arrived yet. Will take some more tests in the morning but are false positives common? Does this look positive or could it be a dodgy test? How accurate are these ones? With my child I only used the clear blue ones.