Sorry to hear you're in so much pain. I have it myself (have had laps and laser surgery and hormone injections etc!) and know how it can hugely affect your life and everyday things.
I think you need to be careful with things like yoga, as they do help for some people, but some types can be quite extreme and actually make the pain worse.
I recommend perhaps seeing an osteopath - i have seen one recently for my lower back and it has really helped me. If you ask them to be gentle and explain how serious the pain is they can really help.
They also will listen to your body as a whole rather than focusing on just one part.
I find that painkillers don't do much if the lower back pain is due to endometriosis, but gentle massage can be brilliant as it also relaxes the body and therefore the pain feels less and means you can get about more easily.
Please don't feel alone - I am in constant pain, which ranges from intense back pain and pelvic pain(which i am feeling so sick from as i'm writing this!) and have had to stop looking for work as the pain is just too much at the moment and i can't commit to social things with friends as i am in so much pain. It does fluctuate over time, though, and nothing lasts forever so don't feel too sad!
Does the back pain come alongside pelvic pain? and does it get worse at any particular time? (during period, etc?)
If so then it really sounds endo related.
But then you can also get referred pain. I get it down my legs too.
I have been told i suffer from IBS too but it all feels like the same endo pain!
Please dont feel alone - i feel your pain and it's good to reach out to others who know what you mean.
Take care and hope you start to feel better soon
x x x