Welcome to HCM,
After going through your history, you are most likely not pregnant as you have period twice and pregnancy
tests are negative.
The nausea and distension of abdomen may due to some other cause like Acid peptic disorder.
If there is any doubt please visit a Gynecologist and get blood Beta HCG
if this is negative means you are not pregnant.
You may also do a sonography
of abdomen to confirm.
If pregnancy is not planned some form of contraception
must be used as it also protects from STDs.
Please take care