This year is a NIDCHEH, which means the fast day was postponed. The actual fast day would have been Shabbat, and we don’t fast on Shabbat, so it was postponed to Sunday.
It has the same level of stringency as the other fast days, like Taanit Esther and Tzom Gedalia.
You should not fast if you are ill, even if it is just a headache.
You should not fast if you have any health issues.
If you are not sure, do not fast.
In regards to showers, they are prohibited if for pleasure, for instance at a spa or just for enjoyment. On any Tisha Be’Av, but especially a Nidcheh, a person can take a regular shower if they are sweaty, dirty, or feel like they are in need of a shower.
Have a meaningful and spiritually uplifting day!