I don't really know why your cycle would change as you described, although I do know it can take a while for everything to regulate after coming off bc. But it's weird that you had 4 "normal" cycles then for it to suddenly change. What does everyone else think?
I read it is possible to have a light af the first month of being pregnant but I am not sure about it happening 2 months in a row. Have you tested to be sure? If not, I guess it must be one of those tricks nature can play on us and your body is still adjusting to coming off bc. I guess this time of adjustment may also be affecting the results you get from your monitor.
Let us know how you get on and if you get any info from your monitor manufacturer, it would be great to hear it!
Good luck
TB