Hello, I Am 24 Years Old. My Last Period Started

Hello, I am 24 years old. My last period started June 28 and ended July 1st. I started taking phentermine around the same time for weight loss. I missed my period for the month of July and I took two home pregnancy tests and I also went to my Drs and took a test all came back negative. Now in August I started to spot one day and that's it. Now a week later I'm stared to get period like cramps but no period. I am now due for my period for this month and nothing.