So I got my first CHs

This is my 2nd cycle off BCPs, but first cycle charting.
I started using OPKs on CD10. CD10 and CD11 were almost stark white OPKs. But the AM of CD 12 was a positive with both lines being equal, my mid-day, the test line was darker, by night it was back to equal again. CD13 in the AM, it was almost back to stark white again. I thouhgt the color was supposed to build gradually? Does anyone else have this sharp of an LH surge? Does it mean anything is wrong?
How does FF decide what day you ovulate? It says I O'd on CD12. I got a pos OPK on that day and had very light dull cramps, but the morning of the CD13th, I had very strong sharp cramps for about 30 minutes so I think that is when I O'd. Who is right?? haha.