I agree with callmemama.
Dd had a few colds where her nose was too stuffy to nurse. Poor thing would nurse for as long as a breath will hold, then take a deep breath, then latch on again to nurse while she held her breath, etc.
I wouldn't worry about your dd weaning because of it. My guess is that she'll want to breastfeed a lot more than usual once her nose clears up. In the meantime, maybe try a saline nose spray or drops (found at natural foods stores and some drugstores) to help it clear up a little (it's not a decongestant, it's like soothing ocean water). And it is definitely okay to offer.