Hi!
Yes, it could be stress and acidity. However, to confirm whether you are pregnant or not it would be better to go for a blood pregnancy test. Please consult your doctor. Take care!
The medical advice given should not be considered a substitute for medical care provided by a doctor who can examine you. The advice may not be completely correct for you as the doctor cannot examine you and does not know your complete medical history. Hence this reply to your post should only be considered as a guiding line and you must consult your doctor at the earliest for your medical problem.
Also I threw up yesterday morning but there was nothing in my stomach because I hadn't eaten since the night before. So it was kinda acidic?