The calcium in tums will not benefit her at all (or humans for that matter.)
You are feeding her what you should be. If you are feeding cottage cheese, that should be all the calcium she needs. She
should be eating about 3 times what she was pre-whelp.
There is a pudding recipe that I know has been posted on here before that you can feed. (mardelin and feminvstr have both posted it before) Also, maybe try soaking her food in the mixture to let it become soggy and soak up the flavor.
Try holding warm compresses to her belly to help draw the milk in. Also, gently massage and rub her belly.
As far as her seeming skinny...I wouldn't be too concerned about that...she just gave birth to pups...there's alot that was in there before that isn't now. But I understand--females can look like skin and bones afterwards.