10 week old and heart murmur

Hi

My baby had her 8 week review and they found a sma heart murmur, she stops breathing when she's asleep for 7-10secs, she hadn't been weighed for three weeks, she only put on a 1lb in 3 weeks, I breast feed her and she will have neocate for top up if required and only if it's necessary as she can latch and keep wanting to latch and makes me sore. I'm starting to worry that she has gained such a small amount it three weeks. My second eldest was breastfeed for a short while and went on to formula neocate as they didn't advise me I could sti feed her. So her weight gain was fine and she didn't have a heart murmur, im starting to worry it could be something to do with murmur or my supplies dropping and she's isn't getting enough from me. I've never been in this situation and I'm scared and anxious as hell.