。
She was crying because it was so painful.
Literal
She [topic-は] painful-was [because-て] [was-crying-泣いていた].
Same つらくて + emotional reaction frame, but with the everyday verb 泣く ('cry') instead of the more literary 涙を流す ('shed tears'). The past progressive 泣いていた ('was crying') captures crying as an extended state, not just a momentary action — Japanese ~ていた is the workhorse for past-ongoing situations, parallel to English 'was X-ing.' Combined with つらくて, the sentence lands as a glimpse of someone in the grip of sustained distress.