Did you increase her food amount while she was nursing? If not, for future reference if you will be raising more baby buns, momma buns usually need to be free fed a good quality pellet and hay while nursing as they need so much nutrition to supply those growing babies. Depending on the litter size, mother rabbits can eat 3-5 times their normal amount of food. If the mom isn't getting enough food while nursing, this means the nutrition will be taken from her, resulting in weight loss and possibly other health issues. If she was being free fed, it's possibly there is an underlying cause for the weight loss, unrelated to the nursing.
Now that the babies are weaned she will just need a normal amount of food to put the weight back on, provided there is no underlying illness that contributed to the weight loss. I would give approximately 1/4-1/2 cup of pellets for a 3-5 lb rabbit, and free feed(never runs out) a good quality grass hay(not moldy, no noxious weeds). It's important to verify she is eating her food well. If she drinks from a bottle, I would provide a water dish as rabbits drink better from a dish and better water consumption will also help with better food consumption.
I would weight her regularly and if she isn't putting weight back on, I would be concerned about something else being the cause of the weight loss, particularly if she had unlimited pellets/hay during nursing.
