A serious concern developing in a new Poly situation

I 32m have been in a 2.5y relationship with 29f (let's call her Sarah), about 1.5y years living together although we aren't living together now. We always agreed to ENM and she had 3-4 short term flings last year that didn't last very long. I appreciated her not hiding and sharing stories with me, and was supportive. Sarah and I had both done std testing at the start of the relationship which came out clean. We never used any protection and always used pull out method.

About 6 weeks ago, I met a 24f woman (let's call her Emily), which have been going pretty well so far. We started having sex around 4 weeks ago (using protection), and both of us got panel tested around that time that came back clean. So around 2 weeks ago, we decided not to use condom with Emily, and I had sex couple times during that week with both partners (separately, not threesome).

This last week is where things started going wrong: Emily had to see a doctor an week ago, because of a vaginal infection, and was told that it could be because of a pH balance issue that commonly happens with a new partner when flora changes. She is taking Metronidazole for a one week course and was told not to have sex for 7 days, which we are observing of course. This was already concerning, until today, when Sarah told me she is also having itchiness and discharge, although she didn't know about Emily's issue yet. She scheduled doctor appointment for tomorrow.

I maintain pretty good hygene, and always had shower before and after sex with both of them. After this scary development, I've stopped having all sex and waiting to see how Sarah's diagnosis and test results come back. Emily didn't share with me all the symptoms and diagnosis, just told me that her doctor told her it's not an STD and there's nothing to be concerned about after the 1 week recovery period.

Sarah feels super concerned that the infection was brought in by Emily, and I can't blame her or disagree. Other than this issue, I really want to continue both relationships for long term, and Emily has been pretty into me so far, but me and Sarah are also quite comfortable and happy with each other.