What would cause a period to go bright red to light pink with blood clots and slimy discharge, and feel tired , and back pain?

Unusual period: What you need to do is do a pregnancy test. If is pozitive check with your doctor. If is negative and this symptoms bother you still see your doctor to find out the cause of it.

Agree with Dr. D.: In addition to the advice from Dr. D., it appears that you are losing more than usual amount of blood. You should take iron and multivitamins to prevent anemia and address your weakness. Have a healthy diet, drink plenty of water, avoid alcohol and street drugs, exercise at least 150 minutes per week. Get HPV vaccine.

All part of normal: These all sound like part of a normal period. Rest and hydrate during this very uncomfortable period.