Being called fat for the first time

My bf and i were walking and I was called me fat for the first time in my life from some random. And I know that this is the farthest from the truth, but it stung. It should not bother me that much and I am so angry at myself for this idiot affecting me.

How do you ignore it or at least not let it ruin your day? I hate it that some random guy could affect my mood so much. It' s so stupid.

For some unnecessary context: it's cold here so i was wearing the chuncky sweater and a bit too large ( loose model and should bought S not M size).