Partners get flu like symptoms but all tests are clear?

Long story short, I slept with this girl a few years ago and got the worst flu of my life 2-4 days after. I suspected an std and throughout a whole year I got tested for everything and it’s all clear.

However, I have had 3 partners since then and everytime we go unprotected, they get severe flu like symptoms 2-4 days after. Everytime I take std tests, bacterial tests, etc it all comes back clear.

What could I possibly have that makes people sick?