EXCLUSIVE

Ben Simmons believes his ex, Tinashe, has been tailing him and showing up at the same places he parties with gf Kendall Jenner ... and it's gotten so bad he's seriously thinking of significantly beefing up security.

Sources close to Simmons tell us ... Tinashe's been suspiciously popping up regularly at places he goes with Kendall ... way past the point of coincidence. You'll remember, the singer showed up at Delilah Thursday, and a couple weeks prior she surfaced at Poppy in WeHo when Ben was there.

We're told Ben is convinced Tinashe cannot accept the fact he ended their relationship and has moved on with Kendall. He believes she's showing up to keep tabs on him so she can see if there's a chink in the armor between him and Kendall.

Our sources say Ben thinks she's become so obsessive he doesn't want to take chances ... especially because of Kendall, and that's why he's talking about hiring extra muscle.