Why did I have my period again right after one day went by?

I doubt you’d see blood so soon if it were a urinary tract infection (UTI) and it would have burned/hurt on your urethra when you peed. If you and your boyfriend had vigorous sex (especially if he is well endowed) it’s possible that you might have just kinda stopped but not totally. Usually if left alone the few days after your period you will have to use a pantyliner because of the brown (dead blood cells exposed to oxygen) discharge where the flow has slowed to where the last bits are leaking slowly. If you had vigorous sex then he could have caused the last bit of flow to speed up. If it lasts more than a day or two go to your doctor and have it checked out. Same applies if you feel light headed, sick to your stomach, or otherwise not yourself.

Usually it’s nothing to worry about. I’ve had it happen to me as well. In the future hold off on the energetic sex till your discharge runs clear/milky white-ish and is back to normal before sex to be sure and ALWAYS use a condom!!! No amount of sweet talk or honey I love you will make up for it if your health is effected. Some diseases don’t kill you but they stay with you forever (such as herpes) so just don’t risk it.