How To Differentiate Between Stomach Ulcer And Pregnancy?

I am 20 years old, 111lbs with a history of stomach problems and vomiting, but I recently (a little over a week ago) had sex for the first time; he used a condom andi am on the pill and now I have stomach pains and nausea. How can I tell the difference between an ulcer and possible pregnany if it's too early to take a test?