What does "beginning" mean in the following sentence?

She will be in Canada next week for the Global Hepatitis Summit. Her patients won't be able to consult with her beginning on Monday, as she will be traveling by then.

Does it mean " Her patients won't be able to consult with her from beginning on Monday" or "Her patients won't be able to consult with her at beginning on Monday" or "Her patients won't be able to consult with her because of beginning on Monday"?

Why is the present participle "beginning" written, not a preposition + a noun(from or at or because of + beginning)?