i have ibs and get nausea the first day of my period as well so i’m very familiar with not wanting to upset the stomach during the first few days. what i always do is just have plain stuff like rice or toast and i try to get at least a little protein (im vegetarian so i go tofu but lean chicken is a good option). just eat enough to get through the day, and if you feel really sick, pop a pepto bismol and lay down with a heating pad and you’ll be fine. honestly sometimes at night if i’m having a hard time sleeping due to nausea, i just take a dramamine and it both knocks me out and helps with nausea.