yes u are verycorrect.
kits are not very sensitive in early days so definately u will have to repeat the pregnancy test.
in case it turns out to be negative ...u should consult ur gynecologist for detailed hormonal essay and blood investigations related to negative periods.
secondly your irregular menses might be due to polycystic ovarian syndrome
for which u will require lifestyle changes and medications.
also these symptoms might be premenstrual syndrome
also consult a gastroentrologist coz it might be gastroenteritis