This is Dr. David
protected sex can repeatedly hit the cervix and can cause the cervix and uterus to contract in orgasm and can contact the next day as well causing cramps
the reddish brown discharge can be old blood in the uterus coming out from your uterus contracting.
if your mild cramps get better which I think they will, this would not be from PID
I'm glad you used a condom.
make sure you urinate after sex to decrease risks for urinary tract infection from sex.
let me know if you have other questions.
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