For the past two days, i've been suffering nausea and a minor, sometime severe, headache. no fever. could it be the flu?

Possibly: According to Mayo Clinic Flu sxs may include “Fever over 100F (38C), Aching muscles, especially in your back, arms and legs, Chills & sweats, Headache, Dry Cough, Fatigue and weakness, Nasal Congestion.” Gastroenteritis symptoms may include: diarrhea , nausea, vomiting, abdominal cramps/pain, headaches, muscular aches & pains as well as fever (low grade). Bloody stools are not usually associated

Flu: It is possible can be the flu, even without upper respiratory symptoms and/or fever. Flu can work in many ways, so be on the lookout for any fever or the start of any cold like symptoms.