why do i feel like i need to start crying during a confrontation?

I ve always been a sensitive person. Everyone calles me a sweetheart because im always nice, thats mainly because i hate hurting peoples feelings and also am a doormat because people use me and i cant say no. Today at work a co worker came up to me and told me she needed to talk to me about something and very aggressively told me about something i did that i wasnt sure about and i didnt say anything back because i felt if i let out one word i would start crying. Why do i always do this i want to stop so i can tell people things without crying, please help