What Causes Headaches Along With Intermittent Nausea?

I stopped taking Tylenol PM when i learned it was unhealthy. Since then i have been sleeping well enough, but waking with a headache. Also get headaches and slight nausea intermittently during the day. Had been using 2 tabs/night for a few years to offset femara and post surgery pain after breast cancer surgery. The Femara (letrozole) has fêwer side effects for me now, and i find the sleep is ok now. But are headaches etc withdrawal effects and of so, how long will this continue? Thank you!!!