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If your baby is 5 months old then even at that time you ma be having lactational amenorrhoea(no menses due to breast feeding).After that you have got Mirena
inserted and then you had irregular bleeding so it is probably due to the hormonal
problem due to mirena .Right now you say you have symptoms of pregnancy
so see if you have nausea;vomiting;dizziness;breast tenderness;low back pain
etc and if yes then get the urine tested for pregnancy.The symptoms like pregnancy can also be due to mirena and the chances to get pregnant are less but since there is a small percentage of failure in all types of contraceptions please get a urine pregnancy test done with the first morning sample of urine for confirmation.