Pregnant ? When to test?

I had a baby 6 almost 7 months ago. I got pregnant super fast for my son. Within a month of getting on birth control. I had my period October 14-17. Me and this guy had sex on October 18th. He didn’t pull out. Then exactly a week later I had this. It only lasted half the day maybe and was very very light. Now I’m having headaches and some nausea and body aches. When should I test??