I would get her in to see a vet as either she is aborting a litter or has a very serious uterine infection called pyometra.
www.marvistavet.com/html/body_pyometra.html
She might be losing mummified pups, placentas or malformed fetuses too. A mummified pup is one whose development stopped during gestation for some reason and the fetus becomes a brownish lump or mass which may be delivered with live pups or aborted.
Whatever is going on I think this female is not one that should reproduce and I think she needs help pretty quick to see what is wrong as if its an infection she could get very ill very fast.
Hope this helps you!
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