How to stop my dog from needing to go to the toilet in the middle of the night?

My almost 4 year old boxer has recently started loosing weight, she went from around 22kg to 18kg in 2 months, she has always been a small framed boxer but has always looked healthy up until this recent weight loss.Anyway after a few visits to the vet where we have decided to try her on a high calorie diet.We started her on 1.5 cups of dry food in the morning and half-1 can of puppy wet food in the evening at around 6pm. Ever since this she has been waking me up at around 2am every single night needing to go outside to do a poo. She used to sleep right through the night and it would even be a struggle to wake her up in the morning.I have tried giving her the wet food in the morning and the dry at night, I have tried feeding her as soon as I get home at 4pm but still she wakes me up at 2am every night.She is perfectly house trained and has had no prior issues with needing to poop at this time of the night. I always take her out at about 10pm and tell her to go pee and she does this with no issues.I am going to Europe in 5 days and I will be gone for 5 weeks and she will be staying with my parents and will be crated outside throughout the night and I am worried about her having to go to the toilet in her crate because nobody will be there to let her out.Am I worrying to much? should she be able to hold it? this has been going on every night for 3 weeks.Thanks