FMLA time

I am currently on intermittent FMLA leave in order to care for my disabled son. My boss is extremely against FMLA to the point of putting a letter in my file for excessive absenteeism. Since I also have fibromyalgia, I use approximately 4 sick days per year for flare-ups. At the suggestion of my rheumatologist, I applied for and received intermittent FMLA to protect myself for any days I am absent due to my fibro. Today I received an email from HR requesting that my son's doctor fill out another recertification form for FMLA due to the "change in days". This form was just filled out in the beginning of May. I don't understand why this needs to be filled out just because I am also on FMLA. Can HR split my FMLA 12 weeks equally between myself and my son for use or is it just a total of 12 weeks. Note that I have not missed any time due to my condition since I have been on FMLA. This is just one more item in a long list of harassment by my employer due to me being on FMLA. I have and continue to document everything because I am assuming they are trying to fire me. HR told me that if there were no FMLA, they would have fired me already, that I was lucky. Thank you.