Hello,
Welcome to ehealthforum and thanks for writing
Since you had unprotected intercourse during your ovulation or fertile days possibility of pregnancy can be suspected.The symptoms as pain in the back and stomach with pink discharge is also suggestive of that.
Now you started experiencing heavy bleeding with clots which can be indicative of possible miscarriage.Considering your age irregular bleeding can also be a premenopausal symptom.
It is difficult to comment without clinical examination and it requires pelvic examination and investigations as USG scan and Hormonal Analysis which would help in ruling out the exact cause.
Kindly consider visiting an Ob/Gyn to rule out the exact cause of the described symptoms.
Take care!