Period Got A Lot Lighter After Period Sex

Okay so me and my boyfriend had sex the first day of my period and my flow was normal for the first day. I'm on birth control so it came on time like to the day and class period (I'm in school) and it's always like clockwork. Well we had sex and everything was normal that night but then the next morning and until now it's been a lot lighter than normal. I read that sex on your period can make it lighter but is it a normal thing and am I just freaking out over nothing?