What Causes Abnormal Looking Vaginal Discharge After Intercourse?

HI There, i have had the mirena for 4 years and 5 months and have not had my period at all. after having sex there 2 days ago there was very strange looking discharge including what looked like a long slimy string. i didnt think much of it but today when i went to the loo (didnt have sex) there was more brown(i assume blood) stringy discharge again. i am just a little worried because im almost at 5 years and have never bled at all andnow theres this strange stringy bloody stuff... should i be worried? has my mirena stopped working? thanks Candice