shannonbaby,
So sorry to hear about that. Don't be angry with your PIL, you shouldn't be. Things are going to get worse if you find blame on someone. You should trust them as the carer, they definitely don't want this to happen also. Cool down and speak to them calmly to find out more about the routine, eg, how often they change the diaper and how soon they change the diaper after your baby poos, etc.
And also to find out the reason for the relapse from your PD. What did he say? I thought cannot stop the antibiotics? My friend's son and my colleague's girl both took antiobiotics for 1 full year to prevent relapse.
Stay strong! Your baby needs you.