if the clumps are small and the pain is mild, then it is most likely just from your period. sometimes the blood gets dark red if it is old blood and stays inside your uterus
a little longer than usual before it comes out. if the bleeding is really heavy and the pain is severe, then you may need to see your doctor to make sure nothing else is going on. sometimes it could be from a polyp or an infection which can usually be easily treated. please let me know if you have any more questions.