Bleeding restarted after sex?

My doctor told me that I could have sex whenever I felt ready. Well, last night at 3.5 weeks postpartum we did and it was a little uncomfortable but not too bad. Well, today I'm bleeding like a medium flow period with bright red blood again. I'm guessing this just means my body wasn't ready and that I should wait a few weeks to try again? Do I need to call the doc?