Two tests....two different answers!

I have been using clear blue digital opk for a while now. Last cycle I had a chemical pregnancy. I started testing again with my CB opk and almost immediately got the blinking face...which continued for 10 days. I got frustrated and went out and got the first response with the lines and started doing them both everyday to compare. Last sat I was still getting a blinking face from CB (neg) and first response was an obvious positive! Confused...so I just kept testing. Today I got a positive solid smiley on CB but first response was clearly negative?!? Anyone know what could be going on???