I had an ectopic pregnancy about 2 months ago. After pregnancy symptoms went away after the surgery I took a pregnancy test and it was negative. Here I am a month later with some of the symptoms returning. My period is due next week and I started the pill right after my surgery. What is the cause of the pregnancy symptoms returning?

Pregnancy and menstrual symptoms are so much alike (for most women) that it isn't meaningful to try to determine which is which.
In addition, starting on the pill will affect your body in similar ways, being made out of the same hormones.
Keep in touch with your doctor, however. You should have been told that the first month of the pill has a higher risk of not being effective yet.