am I pregnant?

I have missed my period by 6 days now. I still have cramps every once in a while, I'm constantly bloated and I have had headaches every day. On the third and fourth day I took First Response pregnancy tests and they both read negative. The label says I could know sooner than 5 days before my missed period but doesn't say anything about taking it after. I'm 19 years old and I heard that my period cycle changes because I'm young so I thought maybe that could be it? Also I read that stress and changing a regular schedule to a new one could have a part in that as well and I just switched from third shift to first on October 18th. Then because my period didn't come my boyfriend and I decided to have sex again maybe 3 days ago but was unsuccessful because I was really dry and vaginal skin came out on him like I would think would happen with a period but just no blood. What are thoughts and what should I do??