Derek Jeter -- Vegas Reunion with an Ex-GF! Not Her, Or Her, Or Her ...

When you've banged as many chicks as Derek Jeter ... running into exes becomes an occupational hazard, and Tuesday night it was Jessica Alba's turn for the awkward moment.

Jeter and Alba were both guest speakers at the Goldman Sachs technology conference in Las Vegas. Jeter was there for his new website (theplayerstribune.com) and Alba was talking about her Honest Company baby products.

The after party went down at Hyde Bellagio Nightclub -- and sources inside tell us Jessica and Derek did have a 10-minute chat while they were there. Nothing hot and heavy, we're told ... just enough to talk about her kids and his retirement.

Still ... Cash Warren and Hannah Davis can't be thrilled.