Tamoxifen can cause nausea or tummy upsets with some people and that also may be the cause of some symptoms.
I had that feeling on taxol, not really nausea just being unsettled (and reflux) it felt a lot like anxiety.
A change in medication regime could easily spike some anxiety and if it continues some sessions with a counsellor may be beneficial.
Whilst it might well be anxiety, the fact it seems to be always there could be more likely to be side effects. Do you eat when you take it? Is it morning or night? Varying things can help. Did you start any other new medications at same time (like iron tablets) always good to exclude other possible causes.