You are negative for pregnant by home pregnacy tests. This test will be positive usually around the expected date of menses.
Since it is negative it is highly unlikely of pregnancy
Ectopi or normal pregnancy, all show positive for HPT.
Ectopic pregnancy may have severe pain
, bleeding etc which will be strong at around 4-5 weeks so you are highly unlikely of any kind of pregnancy.
Your symptoms are probably due to stomach upset
may be a side effect of antibiotics taken for sinus infection
Since most antibiotics causes GI disturbances.
If the diarrhea persist and causing dehydration
you should take plenty of water with salts or take ORS,
with anti diarrheals.
For those cramps you may have anti-spasmodics