I Was Put On The Nexplanon Birth Control About 4

I was put on the Nexplanon birth control about 4 weeks ago, I have been on my period for about 3 weeks now. It started off like a normal but somewhat lighter period, now it's just spotting. But I have felt really nauseous for the last few weeks, so I have taken three pregnancy tests which have all come up negative. Do I have anything to worry about?