Authorities have serious doubts about allegations made by the mother of 14-year-old "Modern Family" actress Ariel Winter ... TMZ has learned -- because they think the mother is making up stories of an illicit sexual relationship to save face after she was stripped of custody.
TMZ broke the story ... a judge determined allegations that Chrystal Workman was physically and emotionally abusing Ariel were so serious, he awarded temporary custody to Ariel's older sister -- and ordered Chrystal to stay 100 yards away from her young daughter.
Law enforcement sources tell us they're not buying Chrystal's claim Ariel was involved in a sexual relationship with an 18-year-old boy, largely because the mom never went to cops until AFTER she lost custody on October 3 -- even though she claims the affair's been going on since the beginning of summer.
Court sources tell us the first time Chrystal mentioned ANYTHING about an illicit relationship was during the guardianship hearing last month ... when she began ranting in court and called the guardianship lawyer a "moron."
We're told she testified about a specific incident on Sept. 24, saying ... "I caught my daughter with her boyfriend in bed -- she's 14. He's 18 and I threw him out and I broke them up."
Due, in no small part, to the timing of Chrystal's testimony ... one law enforcement source tells us they don't believe there was ever a sexual relationship between Ariel and the boy.