Former Disney Star and one of Hollywood's It-Girl Selena Gomez is anxious of Justin Bieber's faithfulness to her as the latter spent time with Victoria's Secret model Barbara Palvin.

Some reports claimed that Selena is shedding weight from her already tiny frame as an effort to make Justin stay. While others claimed that it was because of stress and depression that the actress is experiencing from her relationship issues.

According to sources, the Wizards of Waverly beauty is feeling increasingly anxious about her rocky relationship with Justin despite the pair recently reconciling.

"Her up-and-down relationship with Justin Bieber is taking a toll and she [looks] thinner than ever. She isn't eating because she has so much anxiety about Justin being faithful and doing right by her. Selena's freaking out," a source told Life & Style.

Reports suggest that Selena's biggest worry is Victoria's Secret model Barbara Palvin, who Justin is rumoured to have spent time with earlier this month while he performed at the Jingle Bell Ball in Atlanta amid rumours the pair were having an affair.

Barbara seemed to have no love for Selena as she publicly admitting her dislike for Selena during a web chat with fans.

"She's stressed about the fact that [Justin's] added more tour dates and they'll be apart for longer. She doesn't trust him. Justin is on tour again and isn't living up to his promises to Selena. Selena hates how braggy Barbara is about it all, wearing his shirts, tweeting from his locations, and following him all around," a source said.

However, it seemed that Justin and Selena have patched things up as the couple flew to Vallarta, Mexico and planned to spend New Year there.

The couple each posed separately with a female fan upon their arrival in the sun-drenched country. Justin also posted a photograph of their resort's swimming pool on his Instagram page.

See the slideshow for the photos of Justin and Selena romantic vacation in Mexico.

Mexico Justin Bieber