Can a pregnancy test be inaccurate when..

I'm currently on the mini pill and I'm breastfeeding. I had my period about 6 weeks pp. Then it got light. Now it is gone completely. The thing that gets me is that I am cramping like I would during my period. I took a test the other day and it was negative. Just wondering if being on the pill can interfere with the results? Should I take another with morning pee? A bit worried...my LO is only 6 months. Sorry if this is the wrong forum to post.