This was tough to find.
First, make sure you have "Show ruler" checked under the View menu.
Then, in the ruler at the top of your document, I bet you see something like this:
The little blue rectangle has the tooltip "First line indent" while the little blue triangle says "Left indent". In other words, the first line of your paragraph lines up with the rectangle, while the rest lines up with the triangle. To remove this you need to line them up, like so:
Each paragraph appears to have its own setting, so you'll probably have to select all (or most) of your text and make the change to apply it more universally.
I'm guessing that you inadvertently moved this when first creating the document.
More information on the Page setup support page.