Hi,
Noted the history about your 67 year old sister, lifting of a potted plant, low back ache can walk and lie down with bent legs, took 2 Tylenol, no relief, wants to wait for some time.
As mentioned by you the cause is most probably due to strained muscles that attach the the back to the pelvic bones and sacral vertebrae,
Another possibility is intervertebral joint
arthralgia.
Slipped disc.
local hematoma or so on.
This can be diagnosed by the following ways:
Consult your Doctor for a proper clinical examination so that further investigations as per the clinical findings can be done say for example MRI or so.
The treatment will be specific as per the cause found.
Till then take muscle relaxants.
Ibuprofen if it suits you as Tylenol is
acetaminophen or
paracetamol which is not a good
analgesic unless taken in proper doses.
Take rest.
Apply local rubefacient liniment.
Hope I have answered your query. Let me know if I can assist you further.
Regards,
Dr. T. Chandrakant, General Surgeon