Freaking out about OB -- way too late to change

Just need to vent a little: I'm so stressed out about my OB. She's never been a warm and fuzzy type - she's very direct and no-nonsense, often to the point of being dismissive. But I feel like I've hit my limit with her and am really not sure what to do, at 31 weeks pregnant. After my glucose test, she noticed a dip in my hemoglobin levels and prescribed iron supplements. I started taking them and felt massively fatigued almost immediately (compared to feeling mostly pretty good energy-wise) so I googled around and discovered that -- since I have a thalassemia minor trait, iron pills are not recommended.                           
   
Basically my hemoglobin production is a little defective because of my thalassemia and it can look like iron deficiency when it actually isn't. And prescribing iron pills to someone with my trait won't help and can actually lead to an overdose of iron (spoiler: fatigue is a symptom of iron overdose as well as iron deficiency). I very delicately emailed my OB to ask if she'd refer me for some lab tests to confirm the iron deficiency, and she's already gone into deep ass-covering mode about why she recommended the iron. Not agreeing to give me the referral and stressing me out about it -- I'm not trying to play "gotcha" or sue this woman, is it so much to ask that she show the most basic level of care for me and my child?                                              
31 weeks is definitely too late to look for an alternate OB, right? I've been dealing okay with her fairly dismissive attitude over the last few months but I'm just so frustrated, almost to the point of tears, that she's making me jump through hoops just to avoid taking supplements that could potentially damage my organs. I'll deal with the uncomfortable side effects if they're truly necessary but I don't think I'm a monster for asking for a differential diagnosis when iron is specifically contraindicated for a genetic trait she knows I have.