My teacher added a "because" after ,.
She was in ecstasy when she saw her math test, she received an A+.
You have two independent clauses (each can function as a sentence by itself).
You can join independent clauses with a semicolon: "Eat more vegetables; you'll live longer if you do."
You can use a subordinating conjunction: "Eat more vegetables because you'll live longer if you do.
You can use a coordinating conjunction and a comma: Eat more vegetables, and you'll live longer if you do.
If you use just a comma (as you did) it is a grammatical error called a comma splice.