Well, it might be due to something else... or not.
I had pimples for the first time in about 20+ years after starting tamoxifen, 2 years later they've stopped, so they might calm down. If you think they are pimply in nature, you might want to follow an acne regime. My 12 year old daughters :) is , twice a day,
Wet skin without touching your skin. Very important to be super gentle with your skin.
Then use plenty of cetaphil gentle cleanser. More is more. Use a very gently action, barely touching the skin, to spread the cleanser. Then wash with water using your hands, not cloth, and barely touch your face. Only take about 10 secs. to rinse.
Wait about 15mins for your skin to dry.
Then she puts tea tree oil gel on all the affected areas. Let that absorb a little and then very gently put lots of cetaphil moisturiser on. Again, try not to touch your skin. Just sort of glide your fingers over the moisturiser.
If that doesn't help, you can get a prescription topical antibiotic. I'm starting to develop then opinion that anything you suspect is tamoxifen related should be acted upon quickly, because if it is caused by the tamoxifen, its not likely to get better.