You need to check and ensure this. It is common to run one circuit through to several rooms.
Circuits are load limited and outlets and ceiling lights are very low loads.
Your wiring layout is, as you have found out, a little more compilicated that you had anticipated. If you have the time, you can test all the wires and construct a complete wiring diagram. This would be best done in daylight. I am trying to under the original three wires which feed the outlet. Maybe there was a line from the switch to the ceiling box for future installation of a ceiling fan.