I am 7 weeks post op for elective diverticular surgery. I had a very similar experience to you regarding the exhaustion after using the bathroom. It lasted about 4 weeks. I am 67. I was very tired, and could only manage to get up for the bathroom or to the kitchen to make some soup or a frozen dinner. I had my surgery via robot and the largest incision was dreadfully painful inside like a burning/tearing sensation, I couldn't walk without holding my hand over the incision which helped, but at the six week mark, it started to feel a lot better. I think due to me being alone, I over did it, by trying to do too much after getting home from the hospital. I am finally starting to feel normalish again, and actually drove myself to the grocery store today. Just give it a little more time, and you should be ok.