When two INDEPENDENT clauses are linked by a coordinate conjunction, there is a comma before the conjunction; when there is not an independent clause, independent subject and verb, after the coordinate conjunction, there is NOT a comma before the conjunction.
...I left the office early that day, and I went straight to the doctor.
...I left the office early that day and went straight to the doctor.
...Ms. Hess was elected to the position, and after she was in office for just a couple of months, she resigned.
...Ms. Hess was elected to the position and, after she was in office for just a couple of months, resigned.
When two DEPENDENT clauses are linked by a coordinate conjunction, there is NOT a comma before the conjunction since there is not an independent subject and verb.
...I aided in the process because I felt an obligation to her and because she was drowning in the work.
...We spent a few extra days after the conference ended and after the assessments were completed.
Happy punctuating!
Margie