very poorly dog question please help

Al

I have a king charles bitch who is 6.i have had her to the vets 3times in the last 3 weeks.. She has a problem passing poo.. She can pass it but she screams in pain. She's not coming out of here bed all day every day. Vets have gave her a steroid tablet but she's not getting any better she is so much pain when passing poo. This is so heart breaking for us to watch. Vets aren't sure really what wrong they first thought she damaged her tail. They have told me she can be referred for lots of in depth test. However I don't think this is fair to put my poorly dog through this. Could I have her put to sleep so she's out of pain or should I go ahead with all the tests and scans ext... Thanks