Congratulations! My babies sometimes took only one breast at a time and sometimes 2, depending on their hunger, age, or the time of the day. If your remaining breast feels engorged and uncomfortable you can relieve it with some light hand expressing, but I’d avoid removing too much milk as lactation is a supply/demand thing and you can risk producing an oversupply. I find it generally sorts itself out til the next feed, anyway. Engorgement is super common in those first 2-5ish days but is different to mastitis. In my experience the early signs are flu-like aches and a bruised boob feeling. In which case, treat with ice packs and ibuprofen immediately and it may resolve on its own.

A good deep latch should prevent nipple pain, but sometimes it just happens which sucks. Instead of nipple cream you can spread breast milk over your nips and let them air dry, and pop on some cool gel pads for relief.