3rd Write Up Is This Discrimination?

I was off on disability for a month back in June/July (misscarriage). Prior to being put on Disability I was in the hospital overnight, released on wednesday morning. Put out of work for Wed, Thurs, & Fri. I talked to my boss that Wednesday morning let her know i'd be out the week (I didn't call her I was under morphine, so she called me) anyhow I returned Monday (had dr. app. that day) but I got a write up for missing work the prior wk so I missed my dr. app... Even when I gave a Dr. note!! I was nieve and signed it. 2 months later I got another write up for calling off 3 days. I didnt sign that one becuase I thought it was unfair. Last week I got another write up...for taking 5 day's off since August. I have accrued sick/vacation/and floating which i still have about 40 hrs all together. Now she's trying to force me to sign this 3rd write up, which states if I call off one more time I will be terminated. What should I do, I feel stressed, uncomfortable, like she's always just watching me... =(