PMT is short for premenstrual tension. It names the same unpleasant body feelings and mood changes that many women have before their period starts. This is the name you will hear most in British English; American English usually says PMS instead. Doctors, magazines, and everyday talk all use it in the UK. Tension here means strain in the body and in the feelings, not only 'being tense'. Like PMS, it is a real pattern for many people, and it is unkind to use the letters as a joke when someone is upset. In other contexts the same three letters can mean other things (including a payment), so the sentence has to tell you which PMT you are looking at.