Can sexual intercourse back up your period? And is that normal? Because every time I am intimate with my boyfriend a few days/or week before I am scheduled to start, I always end up being way late. Is that normal? :(

To an extent that is fairly normal. Late periods can be caused by changes in sexual activity and stress. If you are not using condoms, that even adds a whole other factor into the mix (no pun intended). I couldn't tell you if it's common for it to happen every time you have sex at that time, but sexual activity definitely can play a big role in your cycle.