You can test two weeks after sex if you're unsure of when your period will come.

If you are sexually active, it may be helpful to keep track of that. If you have a bad memory (like me!) then write it down somewhere. I make a note on my phone's calendar to help remember. People never look in your calendar anyways.

You should probably look into possible birth control options as well (it can help regulate your period for you!). If you're not into the whole hormonal birth control thing, then remember to use condoms!