My chihuahua mix 7 yr old dog Titus is crying out in pain. He is keeping his tail tucked between his legs and has a hard time sitting. He keeps turning to check his rear end. I know he hasn't moved his bowels in the last 24 hrs. He had no food for approx 18 hr but plenty of water. He will let me touch him around his stomach but cries when you touch around his rear end. Not exactly sure if he is simply constipated or something more significant. It is absolutely pittiful listening to him cry out. Thanks for your help!