I've had my arm implant for about 5 months now. This is my second time on it. For the past two weeks or so I've been feeling nauseous, dizzy, tired, with headaches, my body temperature changes.. And cravings. I took a cheap pregnancy test, came out negative. Is there a possibility test is wrong ? Should I buy a better brand?
PS so to the implant I don't get a period at all. What is the possibility for me being pregnant?

.05% or less. Implants are 99.95% success rate. Doubt you are pregnant. Could be but doubtful. No period, no pregnancy.

Yes, they are very effective, but not 100 percent, While it is posible, it is not likely.
Also, depending on how long it has been since the sexual encounter, pregnancy tests will need to be 3 to 6 weeks after the sexual encounter to test best.

Your signs do sound more like other medical issues, and if they continue, perhaps a medical exam is needed.