Everyone's different. Some people have side effects shortly after starting Arimidex and some people don't have any side effects. My side effects didn't start until I had been on it for several months. They were sometimes mild and sometimes bad. But after a few more months the side effects became worse and my doctor switched me to Femara after being on Arimidex for almost a year. My side effects were insomnia (always bad but got worse), joint pain (I had arthritis before, but I had pain where I didn't before) and irritability (last side effect to appear). I didn't have hot flashes or night sweats, but I didn't after menopause either. Since being on Femara for a month, my joint pain is subsiding somewhat and I'm sleeping better. I took Arimidex at night and take Femara in the morning. I don't know if that made a difference. I'm definitely not as irritable. I lost a little weight after being on Arimidex for several months and gained a couple after starting Femara. Could be the difference in sugar intake.
Some people do better on Arimidex and some on Femara. Hopefully you'll do well on Arimidex!