"Dislocated my elbow last Thursday and not being able to hold my baby is absolutely killing me," Cavallari, 29, wrote on Sunday, her first message since she was hospitalized following an accident in Chicago last week.
(Cavallari and husband Jay Cutler, 32, welcomed daughter Saylor James in November.)
A source told PEOPLE that the former Hills star was struck by another motorist.
"After everything that has happened lately, the accident really shook her up," the source said. "She's just thankful that she didn't get hurt."
The reality star has been keeping a low profile after her older brother Michael was found dead in Utah on Dec. 10. He had gone missing one day after Thanksgiving.
In a New Year's Instagram post, the star reflected on the year, writing: "2015 brought my sweet, baby girl but it also took my brother's life. Hoping for no extreme highs and lows this year. Happy new year, everyone."