Iv Had The Mirena For Nearly 5 Months Now And

Iv had the mirena for nearly 5 months now and apart from the 2 weeks light spotting when it was first inserted I haven't had any pain or periods. Yesterday I had a small amount of bleeding after intercourse which cleared up after a few minutes. But after extreme cramping last night, this morning woke up to find what appears to be my period, however the blood is bright red rather than the usual dark or browny colour. What could be the cause of this ? I do have early stage endo