How can you tell the difference between a pelvic inflammatory disease and an ectopic pregnancy?

I have had pain on my right side like menstrual cramps. It sometimes feels like it is also in my lower abdomen. I have never had pain like this except for when I miscarried my first child. I have nausea, cramps, breast tenderness, vomiting, back pain, fatigue, but it is too early to take a pregnancy test. What should I do? I am so confused and in so much pain.