What Causes Tiredness, Nausea And Lightheadedness While HPT Is Negative?

Last month I had what I thought was my period, it wasn t spotting, but seemed to be normal. It was only 2 days rather than my normal 5 days. Before that period, I had tender breast for 3 days, nausea, and have had felt light headed. I continued to feel nausea and lighthead even after my period. I ve taken pregnancy test, but received negatives. I started getting more tired easily and have loss of appetite. Last week I would ve ovulated again, and have had cramps since (8 days now). My next period would be the 23rd. I m not sure what to think at this point. Any advice?