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Amber Portwood got screwed out of spending time with her son over the holidays, and the kid suffered a vicious dog bite her baby daddy tried to cover up ... or so she claims in legal docs.

Amber says she and Andrew Glennon had a custody agreement in place which allotted her parenting time with their 1-year-old son, James, during Christmas ... but she claims he bolted out of state with the boy without proper notice.

According to docs, obtained by TMZ, Amber says the next shock came when Andrew hit her up on Xmas Day to say James was "nipped" by a family dog and had to go to the ER.

She claims Andrew waited 3 days after that to send her a photo of the injury, and never shared info about James' initial ER visit. In the docs, she says the photo showed James had a black eye and a bandaged nose, "indicating that the injury was more than a 'nip.'"

According to the docs, Portwood is also steamed over Glennon posting a photo of their son on Instagram on New Year's Eve, which she says they agreed was not allowed.

She wants her ex held in contempt of court for allegedly violating the custody agreement. Amber's also demanding more parenting time -- to make up for what she lost over Christmas -- and wants Andrew to cover all her legal expenses.

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As we reported ... these 2 have been locked in a custody battle ever since Amber's alleged machete-wielding incident. She copped a plea deal in the domestic violence case -- pleading guilty to domestic battery and intimidation -- after initially denying any such attack.

Andrew wants to move to Malibu, but she's fighting tooth and nail to prevent that.