My MIL told me to forget about taking a shower or washing my hair one month after giving birth or I will be plagued by headaches and rheumatism for the rest of my life. Any truth to this?

It's a myth. I took shower during confinement and so far haven't feel any side effects yet. Just make sure you blow dry your hair and wipe body and of course avoid direct wind blowing on you after bath to prevent catching cold. If you are sick during confinement you will take longer time to recover as the immune system is weaker during childbirth recuperation. Take a warm drink after bath will make you feel better too.