Hello, I would be happy to help you with your question.
The pain sounds like it was not present when you had intercourse with a condom, so it might be something related to the semen itself. Semen contains chemicals called prostaglandins
which cause contraction of the uterus which might lead to back pain
. This also could be related to an ovarian cyst
. This should be evaluated by your doctor and an ultrasound
is likely indicated.
I hope that this helps and good luck!