Advice please..

I'm irregular as it is, but I have yet to get a positive pregnancy test after not having a period this month. I thought I had positive one but it got voted as a evap. So, I went to the doctor today to see what they had to say. The test I took there as well was negative... But my only other solution is maybe my hormone is super low due to me being on the depo shot? (for six years) And possibly still be pregnant? The doctor said to go ahead and take prenatal vitamins for a precaution. Any advice to help me out?