I am also on Tamoxifen due to my breast cancer surgery in January of 2005. I just looked up the drug in my dandy little book on prescrition drugs. Side effects listed include distaste for food, dizziness, headache, hot flashes, light-headedness, nausea, vaginal itching, or vomiting. These side effects should disappear as your body adjusts to the medication.
The book also mentions that it is extremely important to tell your doctor is you have rapid weight gain (3-5 lbs. within a week).
I would suggest that you contact your oncologist regarding the weight-gain. Cancer is a serious matter, and we survivors should do everything we can to maintain our health.
Breast Cancer Survivor
86 years old