pissed off because my kid won't poop

my patience for this shit (pun intended) is wearing very very thin. DD (3 1/2) has been fully potty trained since november, and has been pooping and peeing like a champ on the toilet since then. there's been some accidents, but it's no big deal; it happens, and she knows that. but all of a sudden for the last couple weeks she WILL. NOT. POOP. she'll hold it on for days on end. i can tell when she needs to go, because she walks away and stands with her butt against the wall and tenses every muscle in her body and trembles and her face turns purple. we go through like 8 pairs of underwear a day because she gets these turtle heads that poke out (hey shadeshaman, perfect example of "touching the cloth"!!) and then she makes them go back in. i have to FORCE her to sit on the toilet during these moments when the poop starts coming out, and she cries and begs me not to make her. i sit with her, hold her hands, rub her hair, tell her she's safe, everyone poops, it's good for our bodies, it's okay to be scared but that nothing bad will happen. i hum with her, mouth wide open, in the hopes that it will help relax the muscles in her body. she crosses her legs and arches her back, ON the toilet, to stop the poop from coming out. after ten or twenty minutes of this i let her stop because it's not going to happen, we clean up her poopy butthole, and she goes to play...until the whole thing starts again twenty minutes later.

we go through this literally every half hour or so all day long, and so do the people who watch her while i'm working four days a week. she starts at a new school next week, what the hell are they going to do when this is going on all day?! her butthole is constantly covered in poop and she gets itchy and irritated from it. her belly hurts all the time. we can't even GO anywhere because it's such a gross battle to have to engage in when in a public place. when she finally does crap every few days it's either in her pants because her body just literally CANNOT hold it in anymore, or it's after thirty minutes of tearful bathroom drama. she feels better after she poops, even admits it. she says her bum feels better, her belly feels better.

she's NOT constipated. she drinks plenty of fluids, gets plenty of fiber, although after holding the poop in for three days i would imagine it's quite uncomfortable, even painful, at that point to let it out. i don't know why she's doing this after months of crapping like a pro, but i know that it's DRIVING. ME. NUTS. the constantly dirty underwear, the constantly dirty asshole, the hours a day spent in the bathroom trying not to scream "JUST FUCKING SHIT ALREADY!"

i know this isn't uncommon in kids her age, but i'm baffled. i don't know what to do anymore. i'm not proud of this-- quite ashamed, actually-- but the other night i got so annoyed around dinner time after two hours of wiping her butt and changing her underwear that i told her to go sit in her room and not to come out until she was ready to poop. she did, looking dejected, and when i went in ten minutes later to talk to her and tell her to come out for dinner she had crawled under the covers, popped her thumb in her mouth, and passed out cold. i undressed her while she slept, tucked her in and told her i loved her, said goodnight, and left the room feeling like the biggest asshole in the world.

making her feel bad about this is NOT the answer, i understand that. but what IS the answer?? is it time to call the doctor? i've tried rewards, a popsicle every time she drops a deuce on the potty. the automatic punishment by default is that we can't even go anywhere lately. it's beautiful out today, we were planning on going to the farm and getting ice cream and feeding the animals, but the poor kid is limping around the house with a stomach ache and a stinging butthole because she has to shit so bad, so it looks like we're kind of stuck here.

please please please give me some advice, some support, anything, before i lose my damn mind.