'Teen Mom' Jenelle Evans -- Goes Back to Jail ... With a Smile

"Teen Mom" star Jenelle Evans can't manage to stay out of jail ... even after copping a plea deal to stay out of jail.

Jenelle was in court Monday for uber-serious drug charges ... including heroin and illegal Percocet possession. A drug paraphernalia charge was added for good measure, because cops say they found wrapping papers and a pipe.

Jenelle copped a deal with prosecutors and pled guilty to the paraphernalia charge. In exchange the other charges were dropped. She was slapped with 18 months unsupervised probation ... but no jail time.

That is, until the judge had her randomly drug tested Monday morning.

Jenelle's attorney tells TMZ ... the MTV star tested positive for marijuana and the judge sent her to the slammer for 48 hours.

Here's the good news ... It takes more than a few days in the pokey to wipe that stupid smirk off her face ... just check out her latest mug (above).