21-50 micromoles per liter (mcmol/L) is the normal range depending upon the lab you use.
Higher than norm ammonia levels are usually due to a problem with protein being broken down in the small intestines. It is then process by the liver and eliminated through the kidneys in your urine. So any problems in this part of the digestive system can result in higher than normal ammonia level.
For example; In patients with advanced cirrhosis, the liver is no longer able to metabolize the ammonia and convert it to urea so the ammonia builds up the in body producing and travels to the person's brain with psychological and other effects.
If you have advanced liver disease you blood levels for bilirubin, creatinine, and INR should be abnormal. Did your doctor perform a "liver panel"?
"Muscle aches and arthritic type pain" are not symptoms of ammonia buildup by the way. Some of the common symptoms of high ammonia levels are...
Changes in thinking, confusion, forgetfulness, mental fogginess, personality or mood changes, poor concentration, poor judgment, worsening of handwriting or loss of other small hand movements, breath with a musty or sweet odor, change in sleep patterns, shaking of hands, disorientation, drowsiness, inappropriate behavior, slurred speech, sluggish movement, unconsciousness and finally coma.
There are also many external factors that can effect ammonia levels.
Eating a high-protein diet
Did you check to see if the antibiotic (or other meds) you are taking increases ammonia levels?
Simple fix: Maybe you are dehydrated? For the kidneys to eliminate the urea they have to be active. The more urine is produced the more ammonia is eliminated.
If you are worried about liver disease or other digestive problem get it an appointment with a gastroenterologist and they can help you diagnose the cause of your higher than norm ammonia level.