well having your mirena out messes up you cycles for a bit. also there may have been nothing for your chemical but you may have ovulated since then and now be newly pregnant. If you are pregnant I have heard several ladies say it causes positive opk's. I would say wait a couple of days and test again on a good brand. BTW I have heard several forums post that first response is the best and that blue test give lots of false positive. Hope you get the outcome you want.
DS born 2003- 39 weeks preeclampsia, 6LBS 7oz
DD born 2009-32 weeks preeclampsia, 3LBS 1oz
DD born 2012-38 weeks, PIH, IUGR 5lbs 4oz- no pree