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?

Okay, sorry, but that sounds like total nonsense. I showered the morning after giving birth in the ward and continued my showers throughout confinement. Sometimes I even took two showers a day. As long as you use warm water and wipe yourself dry immediately, it's not a problem at all.