Is this plagarism? (Maths assignment)
Contact the professor now, if for no other reason than to get out of your panic spiral.
Let them know that you are concerned that you failed to appropriately cite your sources and that you are generally confused about what can be treated as assumed knowledge and when you should credit a source.
You could even go as far as submitting a version of your assignment that does cite sources more completely.
This can be a learning experience. Go ahead and be an active part of that process.