Pains, n.
Labor; toilsome effort; care or trouble taken; -- plural in form, but used with a singular or plural verb, commonly the former. And all my pains is sorted to no proof. Shak. The pains they had taken was very great. Clarendon. The labored earth your pains have sowed and tilled. Dryden.
Shades of meaning
Fernald has no entry of its own for pains; he lists it under industry.
Where he writes about it:
Labor and pains refer to the exertions of the worker and the tax upon him, while assiduity, perseverance, etc., refer to his continuance in the work. — from industry
Pain is the most general term of this group, including all the others; pain is a disturbing sensation from which nature revolts, resulting from some injurious external interference (as from a wound, a bruise, a harsh word, etc.), or from some lack of what one needs, craves, or cherishes (as, the pain of hunger or bereavement), or from some abnormal action of bodily or mental functions (as, the pains of disease, envy, or discontent). — from pain