What Causes Slight Spotting After Intercourse?

Me and my gf been having sex for months now without condoms and I ve been coming inside her and she hasnt gotten her period in a month without getting her period yesterday she thought she got it and today in the morning she went to switch her second tampon but there wasn t no blood on it and today she was leaking liquid that s was a little red but idk what that means and we where soo happy we both thought we where going to be parents but idk anymore we don t want to put our hopes up can you tell us what it is