Throwaway account and first time poster so apologies if this isn't the right format/topic.

I (32M) am currently in a long distance relationship (29F) (talking different continents) and meet every 2 to 6 months. This has been going on for over a year now.

I have had an std check as part of a clinical trial back in April this year. I had just spent two months over a 4 months period with gf. Due to the long distance thing we are highly sexually active when meeting.

The STD check did come back negative for everything.

I recently went back to see her for a month after 6 months, and a month and a half after my return, gf says that she has tested positive for syphilis. So I do get another test and it just returned negative.

I can't find of a way to find out with certainty and narrowed it down to the three most likely scenarios:

1.I am VERY lucky and she had it for a long time and I dodged it (unlikely)

1. She cheated after my last departure hence why she is positive and I am not (quite likely)

2. She may have contracted it by other means (not impossible)

I am wondering whether there's a way to eliminate one or two of the scenarios, or if there are other possibilities to be considered.

Gf does not strike me as a cheater and I had no suspicions prior to this incident (beyond the fact that long distance relationships are hard) but obviously it doesn't mean she isn't.

Appreciate any help/ideas or reasonings.

Thanks.

TLDR: Gf tested positive to STD when I am clean. How to be completely certain she got it through cheating?