I'm a bit confused with what is going on with my body.

I got AF on the 7th of this month which lasted about 5 days.

I started testing for ovulation on the 15th (so on cycle day 9) as my last cycles post miscarriage have been a bit short, I didn't want to miss ovulation.

Yesterday (so the 18th) I got the darkest line on the OPK yet but it still wasn't as dark as the control line.

Today I've done two OPKs & they are lighter than yesterdays.

Tonight I've found blood in my undies - I'm on cycle day 13.

I don't know what is going on! Is this ovulation bleeding, and if it is how come it tested negaetive on the OPK? Could I have a 12 day cycle?