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.