5 day old bunny not breathing right

Hi. My Polish rabbit Lina just had a litter of 6 kits. 4 didn't make it. This was her first litter and I believe she is around one and a half years old. One of the two kits that made it is about half the size of the other one and it isn't breathing correctly now. It was doing fine but when I just went to check on it it would stop breathing then take a deep breath and breath somewhat quickly, then stop, and repeat the cycle. The bunny was otherwise doing fine. A full belly (he doesn't have many siblings to compete with), long hair (much longer than his brother's), and he was breathing fine a couple hours ago. This is Lina's first litter. Please help I've grown rather fond of this bunny.