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Maybe they were picking up normal contractions or thickened bowel loops on the x'rays.
You are describing colitis, which means inflammation of the colon/large bowel and can have many causes Parasites like whipworm, protozoa (coccidia, Giardia), bacteria (Salmonella, Camphobacter, etc), eating something new and stress are some of the possible causes.
Treatment includes restricting/withholding food (but not water) for 24 hours. Reintroducing a bland diet is often suggested (rice and boiled chicken for example) and changing back to the normal diet slowly over a couple days once the diarrhea has resolved.
Peptobismol can be used for non specific treatment. The dose would be one tab of the regular strength pills or a child's dose of liquid. It will turn stool black, which is not a problem, but should not be confused with blood.
If severe diarrhea or vomiting starts, emergency vet care is indicated.