Rambam in Chapter Seven of Shemoneh Perakim writes:
It is not necessary for a prophet to have mastered all the ethical virtues to the point that he does not possess any shortcomings at all.
(Touger translation p. 43)
He then provides various examples of prophets having shortcomings, the last of which is:
And Jacob feared Esau.
Sanhedrin 38b contains a chronology of Adam's first day, in which the births of Cain and Abel precede Adam and Eve's sin.
I think the implication is as you suggested, that those births would not have been as painful as those following the sin.