Since I don't know the date of your last period or how long your cycles usually are, I can't put them into a pregnancy test timing calculator. If you'd like to do that, there's a really awesome one over at https://countdowntopregnancy.com.
Now, since you had unprotected sex about two weeks before your period was due to arrive, it's entirely possible that you did get pregnant. Most women ovulate around two weeks before the beginning of their period, and this is when we're most fertile.
I'm not sure when you took the test and what kind you took, and it IS possible to get a false negative due to user error. I've done it; taking one too early during a month that it turns out I was actually pregnant.
If I were you, I would buy another pregnancy test and take it right away. Since your period was due at the end of March, you're probably fine to take any brand at any time because if you're pregnant, there should be enough pregnancy hormone to get a positive test by now.
My personal favorite for testing is just a good ol' First Response with the pink lines, not the digital.
However, if you're nervous about interpreting the lines, you might want to opt for a digital which will give you a clear "YES" or "NO" answer.
If you take one and it still comes back negative, then take a deep breath and remember that there are a lot of totally innocent reasons that a period can be late without being pregnant, including just ovulating a little off schedule. If your period ends up being more than two weeks late and you still have negative tests call your doctor and let them know. They may not do anything until you've missed at least two periods, or they may have you come in to explore other reasons your period could be late.
Apart from pregnancy, a late or missed period isn't usually serious at all.