Whoa!!! Tyga flew solo for his birthday last night -- "girlfriend" Kylie Jenner was NOWHERE in sight!

As X17online reported on Monday, Kylie looked downcast after spending the weekend away from long-time boyfriend Tyga. She sought solace in a male friend over lunch in Calabasas.

Then, Kylie and Kendall jetted off to Australia to promote their new Topshop line -- putting her out of Tyga's reach until yesterday, when they returned.

The pair haven't even visited since Kylie's return and something SERIOUS happened because she skipped her homie's 26th birthday bash last night at The Nice Guy. NONE of the Kardashian-Jenners were there and frankly, the night seemed super boring -- with a bunch of dudes walking around in "Twelve" caps and slapping each other on the backs while they downed Remy Martin.

So just what happened? Well, some outlets are reporting Kylie's upset after being reminded of Tyga's alleged affair with transgender model Mia Isabella.

After Charlie Sheen announced this week he's HIV-positive, Kylie did the sexual six degrees of separation math and traced Charlie's ex, Capri Anderson, to Mia -- the two were roommates back in the day. There's reportedly a sex tape with the two "girls" -- meaning, the potentially infected Capri could have infected Mia, who could have infected Tyga. Which means Blak Chyna, their son King Cairo and ... KYLIE could have been exposed. Is THAT was the breakup's all about???