It may have just been the stress of you being gone that could possibly be causing the poop changes and the lethargy, but the parasite infection coccidia can also cause a rabbits fecal poop to become deformed and misshapen like that, and coccidia can be a fatal illness. If your rabbit isn't already feeling better at this point with his poop back to looking normal, I would get him to a rabbit savvy vet today to get checked and have a fecal float test done to check for coccidia.
If he has coccidia, the med the vet will put him on will usually be ponzuril, albon, or smz/tmp. You will likely also need to start syringe feeding regularly throughout the day if he isn't eating well, and the vet can give you a syringe feeding mix for rabbits so you can feed him. Coccidia is considered an emergency, so mention this possibility, and your rabbits lethargy and lack of eating when you call in to make the appointment. With coccidia, he would need to be seen today, not later in the week. If he doesn't have coccidia, the vet will likely give you some meds and feeding mix to help your rabbits digestive function get back to normal and your bun feeling better again.
http://www.medirabbit.com/EN/GI_diseases/Protozoal_diseases/Cocc_en.htm
https://rabbit.org/vet-listings/