Britney Spears Explains Why She Became More Rebellious Online

Britney Spears says there's a reason her Instagram looks more rebellious in recent years ... tracing it back to an alleged theft at her home that she says left her feeling violated and completely powerless.

The pop star unloaded in a lengthy post Sunday ... claiming 3 years ago coats, jewelry, and roughly half her wardrobe disappeared from her home. Britney says the response she got afterward was essentially that nothing could be done ... and she says that experience hit her harder than people realized.

According to Britney, the alleged theft "100 percent hurt" her emotionally and left her feeling helpless. She says, looking back now, the incident may have fueled the version of herself fans have seen online ever since ... admitting she "100 percent rebelled" on IG and probably came across as "cheap and probably secretly angry."

Britney even questioned whether the person responsible knew exactly what they were doing ... suggesting the loss affected her on a deeper level than she initially understood.

Britney's been through plenty since the alleged theft. As TMZ previously reported, she was arrested on a DUI charge back in March when cops reported pills were found in her car ... she then checked into rehab and ultimately pled guilty to reckless driving involving alcohol -- and she's on probation for the next year.

Now, Britney says some of the behavior fans have been watching play out online may have been rooted in something much more personal ... a feeling she still doesn't seem to have fully shaken years later.