12 dpo and feel like AF is coming

[military]

Hey maybe someone can help me. I've tried two different ovulation online calendars, both have identical info but one says am 10 dpo and the other says 12 dpo and confused on when to test, today am having the same feelings in my stomach like AF will be here in one second its not cramps. I don't have any other symptoms. Don't know if is implantation or AF. HELP