Could I be pregnant?

Ever since coming off of the pill, I've been getting my period about 14- 16 days between each other. On the 27th and 28th, I had spotting and brownish dried blood. I've been feeling very nauseous lately and feeling indifferent towards food. But besides my missed period that's the only symptom I've noticed that I could be pregnant. However, I took 4 pregnancy tests on the 1st day I missed my period, the 7th, 10th, and 14th of August. They all have came back negative. Is it possible that I am still pregnant even with the negative tests? And when should I test again if i still don't get my period.