My dog wont go to the bathroom and seems like he's in alot of pain, what can i do?

My dog, Oreo, wont go to the bathroom and seems like he's in a lot of pain. He has had back problems before but we just aren't sure what the problem is. He walks really low to the ground and stops to sit as much as possible. He also walks really fast, so he can get to the carpet or inside. (He's not an outside dog, this is when we take him outside) And he wont walk without a leash and we're not even pulling on him, he's willing to go with a leash. And we just had him at the groomers, and at my grandma's house, but we are leaving again today (our schedule isn't always this busy) And I also think he has anxiety issues with us being gone. He is a little black and white Shih-Zue and is very loving, indapendent, and..... cute. lol. But we have tried vegtible oil (which he liked, and i have always thought that animals choose the things that will make them feel better) and it seemed to perk him up for about an hour and then he went back to moping and whining. We also gave him Cranberry Juice and it hasn't worked AT ALL! PLEASE PLEASE help us!