Blames Positive Steroid Test On Pork Burrito

Shelby Houlihan -- arguably the USA's best female runner -- is insisting she is NOT a cheater despite a recent positive steroid test ... claiming it's all a mixup caused by a pork burrito.

The 28-year-old Olympian spelled it all out in a social media post Monday ... explaining she believes she's been wrongfully hit with a 4-year suspension due to a trip to an "authentic Mexican food truck" in Oregon several months ago.

Shelby says back on Jan. 14, the Athletics Integrity Unit informed her a sample she submitted on Dec. 15, 2020 had contained the banned substance Nandrolone.

The anabolic steroid is considered a performance enhancer, and Shelby says she was immediately suspended for having it in…