Does this happen to anyone else? is it normal?

Keiry
I've had my mirena for 3 years now, and my last two periods I've been bleeding extremely dark "old" blood. Should I be worried ? And could it be related to my mirena? Would it affect my fertility? I'm scared I might not be able to have another baby if it's an infection with my mirena. Does anyone else with a mirena have the same issue? Is it normal?