Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today. I do apologize that your question was not answered before. Different experts come online at various times; I just came online, read about your situation, and wanted to help.
Now I share your concern about Abbey.
Just to comment on your mention of her lymph nodes, it is important to know that the lymph nodes do not need to be up in all forms of cancer. They often are elevated in lymphoma cases but even then we can see a lack of enlargement of external nodes while the GI or internal nodes are affected by the tumor. Therefore, if we have gut signs, belly distension, and a cancer worry, then it'd be worth speaking to her vet about an ultrasound. This would let you see inside and determine the root of her issues. You will be able to detect if she is actually constipated or if there is fluid compressing her GI (making it hard to pass stool) or changes/thickening in her intestines that are preventing her from passing normal stool. Depending on the findings of these, you will be able to appreciate what is triggering her signs and what can be done to help her. As well, it may also be worth having them examine those white blood cells in her blood sample to make sure they are not cancerous in nature.
Finally, just in regards ***** ***** GI signs, if the antibiotics are making her nauseous then you can speak to her vet about dispensing oral anti-nausea treatments (ie Cerenia, Metoclopramide, Zofran) or you can even try her with an antacid (ie Pepcid, Zantac). As well, if she has straining to pass feces, then use of a cat hair ball treatment or a GI lubricant (ie Lactulose, Miralax, food grade mineral oil) can be used to ease her passage of stool.
Please take care,