leaking when breastfeeding

My newborn & I have been working on breast-feeding. She had a short stay in the NICU after being born and was fed formula and some of my colostrum. For several days after being home I fed her with a bottle of pumped breastmilk. As of yesterday I've been able to feed her almost entirely off the breast but I have to use a nipple shield when feeding her. I alternate breasts and feed her in belly to belly style. I've noticed that a lot of milk will leak out the side of her mouth as she's feeding. Is this OK? Is she still getting an adequate supply of food/nutrition? Our average feeding time is about 15 to 20 minutes & she'll stop/doze off. Will I eventually be able to feed her without using the nipple shield?
Thank you! 🙂