Cases of nausea started last mon after my period ended sun, i've been sexually active but i'm on ortho tri-cyclen lo. is this just a stomach bug ?

See a doctor: It is not clear from a description of nausea alone what it may be due to. This symptom occurs with such frequency in association with literally dozens of conditions that trying to attribute it to one without a history and exam is virtually impossible.

Possibly: Nausea is a common symptom for many illnesses. There can be pregnancies even while taking birth control pills, so you should check a pregnancy test. If it is negative you should be evaluated for other causes of nausea.