yes you are at risk for pregnancy. changing contraceptives is a change of hormones (and all birth-control works differently) so while mirena may have worked by physically blocking sperm and thickening cervical mucous and hindering reproduction that way this new birth control may need time for the hormones to get into your blood stream. test for pregnancy when you get the chance and proceed from there.
removed mirena started birth control pills same day. had unprotected sex 3 days later. at risk for pregnancy?