Bra sizing?

Alright so for years I wore 34B. I resized a year ago as 32C. But 3/4 of the bras I bought at that size are so tight. They leave deep red marks on me at the end of the day and I dread putting on a bra in the morning. I've been resized again and again at LaSenza and VS as 32C but these bras are so uncomfortable. I figured maybe the clothes I'm wearing and the possible idea that I'm currently wearing the wrong bra size keeps skewing my results. I keep trying to size myself with tests online and I get results from 36AA to 32C and even 36DD. Can anyone help me find out my size? My over bust is 34", bust "35 and under bust 31". I appreciate your help.