Suddenly crying to sleep

Our little girl was born full term, Feb 1st, and has had issues falling asleep for a while, and things got better when we put her in her own room and extended her sleep routine...only fussed for 15 mins or so in her crib and would eventually fall asleep. Now, suddenly, we seem to be back to the crying she did when we started full extinction sleep training at 3.5 months...full on crying for 30 mins or longer before falling asleep. We're so miserable listening to her cry, but we know that if we try to soothe her, she might develop a sleep crutch and also she already strongly resists soothing before we put her in her crib and has for a long time now, arching her back and pushing away when we try rocking or cuddling her, so we know that won't help get her to sleep. We just don't know what to do. She gets more than enough sleep throughout the day/night, and we try to put her down before she starts rubbing her eyes so she's not overtired, but the crying just breaks our hearts. Has anyone else been experiencing this?