Which bra size?

Ok so my junior year of high school, I wore a 36C, but I was starting to think I was a D, but I just shrugged it off, thinking bras were SUPPOSED to be uncomfortable. Senior year I wore a 38C, but then I did research and realized I had all the signs that I was wearing the wrong size so I tried out oa 36D and it felt WONDERFUL! I couldn't remember the last time I actually felt good in a bra! the only problem was the band slipped down a bit so I tried on a 34D but that was too small so I just bought the 36D.and now I wear that but the band still slips down so I'm Now wondering if I an in fact a 34E. I don't know where to get fitted and I don't really want to...