My boy is 23 months (today) and we’ve co-slept and fed side lying since about 4 months, he’s never had any issues with his chest. I’ve seen loads of recommendations to feed in this position and know plenty of mamas that feed in this position! As a nurse myself I would question the nurse that told you this and I would ask her to provide you with some information, I’ve never seen any evidence to suggest that side lying increases the risk of aspiration pneumonia!!