Former "Malcolm In The Middle" star Frankie Muniz had a 'mini stroke' last week and doctors are stumped as to why it happened...

image: getglue.com

Sources close to Frankie tell TMZ, the 26-year-old was taken to an Arizona ER on Friday when friends noticed he was "acting really weird" -- having trouble speaking and understanding words -- and they got freaked out.

Doctors told Frankie he had suffered a "mini-stroke" though they didn't know what caused it. Frankie's still awaiting test results.