Hi, thanks for your question.  It may be a simple difference of positioning with different partners. When a woman is underneath with legs bent back for deep penetration then there is a lot of contact with the cervix and a lot of pressure on her lower back.  However, it’s important your partner consider seeing a doctor for a medical assessment about these symptoms.  Testing like blood work, swabs and/or and ultrasound may be part of finding out what is causing the pain.  Consider taking penetrative sex out of the mix until she can see a health provider.
When you are in an open relationship, it is a good idea for both of you to do routine STI testing every 3-6 months to have current negative test results and treat any STI’s that might be transmitted through sexual contact with sex fluids even when condoms are used.  If an STI infection is detected, then you can be reassured that you have not had it for very long ( based on your last negative test results) and be able to get treatment right away for everyone.  Having the discussion with your outside partners about  their testing for STI’s  is recommended.  Hope this helps. Health Nurse