Confused! Help!

Mackenzie
Hi everyone. I posted a couple days ago pics of two upt's I took on Thursday (2 days ago) asking opinions. Everyone said they looked positive, and my mom (who works in an OB office) said the doctor there looked at the pics and also confirmed that they looked positive. 
Friday morning, I take two more upt's, the same two brands from the same boxes I got the previous day. They were both negative (well, maybe a tinyyyyyy line on both, much much fainter than the previous day). 
This morning (Saturday), I took one first response upt and it also looks negative to me. Does anyone know why or how this could happen? 
My period is supposed to come on Monday. I haven't had any bleeding or spotting. Last night I had a crazy migraine and threw up 3 times. I haven't had a migraine in over 2 years and I've NEVER had one that made me throw up. 
Thoughts or opinions? Could I still be pregnant?