Did you check to see if he was dehydrated?
Can you show me a copy of the blood tests? It will help to know what was (and was not) tested for.
Is Bailey a girl or a boy? Sometimes you say she and sometimes he. If a she, is she spayed? There is a problem that could explain these symptoms called a pyometra that happens in unspayed females.
Did you do the test for dehydration? If you check Bailey's gums, are they pink in color or do they seem pale?
Since I can't examine Bailey, let's make a "problem list" to help me try to tell you what could be wrong. From what you are telling me, her problems are:
Weak; falling down
Sleeps a lot
Is this correct? Based on that there are many things that could be causing these symptoms in a 16 year old dog: Anemia (low blood counts), pain, pancreatitis, liver or kidney disease (even though blood tests 3 weeks ago were normal, these are still possibilities), cancer (blood tests tell nothing about if a dog has cancer), inflammatory bowel disease.
To tell you for sure what is wrong with Bailey, I would need to see the blood tests, and do a physical examination: check the level of hydration, palpate lymph nodes and internal organs. feel the joints and back and spine, listen to heart and lungs with a stethoscope, take the temperature, check the color of the gums, etc.
Bailey sounds sick, painful, and possibly dehydrated and probably in need of medical care.