Knowing your cycle with Mirena?

When I was younger I was very athletic and would 50 to70 days without a period. Then I took ortho tri cyclen lo and had regular periods for four years until I started having hormone problems and side effects so I got Mirena. I've had it for about a year and a few months but once I got it I lost my period and now I don't know when my cycle begins or ends. How do you tell?