Sounds hormonal and can be a shock! Is it progesterone phase of treatment? Many ladies find this difficult to tolerate - someone with knowledge will be along soon.
I know when I had Tamoxifen, within 24 hours of the first tablet I felt like I had a bout of 'flu and it shook me to the core. After 4 months of dreadful symptoms I stopped the drug.
Do you keep a mood/symptom diary?