Because your body was done bleeding..and it was just a coincidence that you had Sex that night. The one doesn't affect the other. After my m/c I bled for 5 days..so bleed for 2 others for 2 weeks, others none at all, it just depends. Your body was just done with it's bleeding, thats all-it means nothing more.
Then why it stopped then?
thats not the reason why your bleeding stop
The only way to know is the do a HPT but it has to be 2-3 weeks after intercourse. If you are asking if the thats way you stop bleeding, then No.