About a month after I potty trained DD2 she got a horrible stomach bug during which I was up to my ears in sick and diaorrhea for about 4 or 5 days. After this she had a total relapse and refused to poo in the loo. I found it VERY frustrating as she'd been absolutely fine before so I knew she could do it. I made far too much of a big deal of it, constantly asking her if she needed the loo, forcing her on if I thought she needed it, bribing her, etc etc, all to no avail (she'd run off and hide behind a curtain 30 secs after I'd taken her off the loo and poo in her pants). On various occasions I had to have her in nappies again if I was out and accidents were an unacceptable risk. At those times she never asked for the loo (even for wees which she was still fine with when in pants - she just happily used the nappy).
ANYWAY, after over 2 weeks and much deliberating about whether to go back to nappies full time (next baby due shortly and I wasn't coping well with pooey pants), I changed tack completely. I never mentioned the loo, never asked her if she needed it, and when she did a poo in her pants restricted myself to saying something along the lines of 'Oh poor you, never mind let's get you cleaned up, next time I'm sure you'll get to the loo in time'. Once the pressure was off her it was miraculous, 3 days of this approach and then suddenly she said she needed a poo, sat on the loo and hasn't had an accident since (a few weeks now).
It's a difficult approach as it's hard to hold oneself back, but it really worked for DD2. She knew what to do and didn't want to poo in her pants, I think she just found the whole thing a bit daunting and too much pressure and expectation from me and once I stepped back completely and let her take control she sorted it out. Anyway, you could maybe grit your teeth and try this approach for 3 or 4 days - expect lots of accidents and treat them with no comment except 'poor you, better luck next time' and see what happens? Give yourself a break for a few days with her back in nappies then give it another go.