Confused

I made a previous post about dome of my symptoms. My older sister is an RN on an army base in Germany. I described my symptoms to her and she keeps telling me to wait until July 30th to test for pregnancy. But it seems a little drawn out to me. My period comes every 21 days give or take a day. This is the first time I've experienced this. Maybe it's me over thinking it. By the time the 30th comes, I would've missed 2 periods (possibly). I would like to know because I am turning 21 soon, not that I have any birthday plans. But I do wanna start making arrangements because I am in college and we can't live on campus if we are expecting. I've paid for my dorm and if I wait too late I can't receive my $5,000 for my dorm room. Any advice? Cause my symptoms get worse by the day and I just don't know what to follow.