It could mean a number of things ranging from there simply being someone behind you, to a guilty conscience, all the way up to full blown psychotic delusions.
Personally, I’m quite a generally paranoid person, and I’m hyper-conscious of what goes on around me, particularly when it comes to people so I’m constantly checking what’s going on around me and my mind is analysing 24/7 . It’s to the point if I’m around people I sit with my back to the wall so no one can be behind me, and so I can see what everyone around me is doing.
But seeing as you didn’t mention anything about there actually being someone behind you, and considering if you had been suffering delusions you would likely know it, it sounds like a mix of paranoia and a guilty conscience.
You have to work on why you think there is someone behind you, then work on why that idea bothers you so much/so what if there was?/what would you do if there was?
In answering those three questions, you will likely find your answer.
All the best.
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