Any thought or suggestions???

I have a 13lb Flemish giant mix (I think with Palomino). We do not know her age and are guessing she is 6 years old.
This past week she has started favoring her one hind leg and stumbling when she tries to walk or hop.
After taking her to the vet and getting x-rays and blood work done she is on Metacam for pain and it does not seem to be helping and she seems a little worse.

The x-rays and blood work all came out great showing no issues.
They said her back legs were weak.

The only suggestion they had was to get a MRI done to check her spinal cord for damage.

Does anyone have any suggestions or experiences like this?

We were giving her Arnica Montana 30c which is a homeopathic anti-inflammatory and that seemed to help but the vet said don't use it since there are no studies on use in small animals. My friend uses it on her dogs and I'm really tempted to go back to it because it helped.

I've got one on joint supplements but I know she won't be here much longer--she turned 18 in July. Some seem to age quicker than others. I'd check with another vet as MRI is not cheap--it would rupture my budget.

What supplement is yours on?