Oh poor little man! How old is he? So my DS did get sick in his cot when he was about 1 1/2 and I felt exactly the same as you. I did go in there, not because he cried exactly, but I heard him sort of grunting through the monitor. When I went to check, he had been sick (quite a lot too),and he wasn’t fully awake either.
This only happened the one time, but it put the fear of god in me. My son also had oesophageal issues until around age 2 and would often bring up some food/milk, but only this one incident at night. I slept on his floor for a while after that, just to put my mind at rest.
The most important thing is, he is okay. You are a caring, loving Mother and sometimes guilt is a by product of that, unfortunately. Maybe camp out in his room, or set alarms on your phone so you can check in, if that helps put your mind at rest.
Also, you can order a bendy arm monitor holder from Amazon, so you can get the full view of his cot. But that depends on his age (my son started to pull the bendy arm as he got older).
Oh, and my boy didn’t care about having a full nappy either. It’s only when he moved into pants at two and half, he started to feel uncomfortable with wet.
Hope you’re okay Mumma x