I understand that Maggie was given more treats than usual, some she had not had before and then vomited with some red tinged fluid, and you are concerned.
It is entirely possible the red color was dye from those treats.
Sometimes if a dog vomits forcefully, or eats something irritating to the gastrointestinal tract (like plant material), the lining of the esophagus or stomach gets irritated and the small blood vessels in the esophagus or stomach break and thus we see blood or a pink tinge in the vomit.
If her gums are a nice bubblegum pink color then the bleeding is very likely very little.
There may other causes as well.
In a younger dog I would be concerned about bacterial, viral or fungal infections, her eating something she should not including a foreign body (rubber band, bones, or toy piece) or inflammatory bowel disease. Other less likely possibilities at her young age are internal organ (kidney or liver) disease that cause a build up of waste products which the organs normally remove that can irritate the gastrointestinal tract leading to bleeding or pancreatitis, or infiltrative cancers like lymphoma.
Gastrointestinal parasites are rarely the cause of vomiting but can cause gastrointestinal bleeding.
Does she have a fever (rectal temperature greater then 103F)?
I would start with an acid reducing medication to try and settle her stomach. You can give either:
1)Pepcid ac (famotidine) at a dose one 10mg tablet per 20 to 40 pounds of body weight every 12 hours
2)Prilosec (omeprazole) at a dose of one 20mg tablet per 40 to 80 pounds of body weight every 24 hours
These are acid reducers which will decrease irritation of the stomach and should help her feel better. They can be used for several days if need be.
Then I recommend a 12 hour food fast. Water is fine as she needs to stay hydrated but by taking away food we rest her gut and hopefully reset normal gut motions.
In 12 hours start a bland diet of 1/3 boiled, skinless, white meat chicken minced or boiled, very lean, ground beef mixed with 2/3 boiled, plain white rice or pasta. This is an easily digested bland diet and won't irritate her stomach and intestines.
Feed the bland diet mix for several days until her nausea clears, then slowly start mixing in her regular diet. It should take about a week to convert her back, mixing a little more regular and less bland in each meal.
You can also give her probiotics to replace good bacteria in her gut. Fortiflora and Benebac are very good. Your veterinarian or many larger pet stores carry them.
If even with the acid reducing medication, her food fast, probiotics and bland diet she still is vomiting and/or refusing to eat she should be seen by a veterinarian. If she continues vomiting blood even with the acid reducing medication then she needs to see a veterinarian on an emergency basis, otherwise she will become dehydrated.
Please let me know if you have any further questions.