Pregnant with Mirena IUD?

Ive had the Mirena IUD for 8 months now, & a regular period every month since then. Except this month of June. No period, but slight cramping & lower back aches, & some headaches along with slight nausea. I know that its very unlikely that Im pregnant, but Im still concerned seeing as I havent been able to locate my IUD threads! I took 2 hpt, both came out negative. Something just doesnt feel right! Is this normal to just stop having a period, & acquire these symptoms? Or is there a possibility I can be pregnant?