Being told I have to cover up

MaLove07

Long story short, I was flying with my DD and a flight attendant told me I had to cover up (I was in a row by myself and was wearing one of those nursing tanks with two layers, so my nipple was literally just sticking out of a little slit in the fabric).  I am writing a letter to the airline to complain, but who should I send it to?  I was planning on sending into the address they list for complaints, but should I also send it to the President of the Board of Directors?  Or is that overkill?  This has my blood boiling!