I keep feeling my stomach it feels like a heartbeat but i don't know if i am pregnant cause i have the mirena (levonorgestrel) in my uterus ?

See a doctor: there is so many normal explanation for that feeling but it have to be confirmed by physical examination. so go and see a doctor for a proper evaluation.

Unlikely: With a Mirena (levonorgestrel) IUD it is very unlikely that you are pregnant and the fetal heart beat can't even be heard with a fetascope ( a special stethoscope ) reliably until about 20 weeks much less felt. If in doubt you can always check a pregnancy test. Talk to your primary care provider about what else might be going on. Hope this reassures you some.