Welcome to JustAnswer! I am a licensed veterinarian and would be glad to help!
An ALT of 125 quite honestly is NOT that high...just out of the normal range, and alone - without other bloodwork changes is not a sign of obvious, priXXXXX XXXXXver disease.
With that said, based on the age, breed, and signs - an endoscopy would not be the next obvious step based on your history.At this time, I would consider an abdominal ultrasound. This will give you a completely NON-invasive and easy look at all of the abdominal organs - the stomach, intestines, liver, spleen, pancreas, etc.That will help determine if there is a belly infection, inflammation (such as pancreatitis) or other intestinal illnesses which can explain your signs and better help direct treatment.If you are not familiar with this procedure, this link (CLICK HERE) will help.
Is it possible that chewing grass right after a chemical treatment could cause this type of reaction that could last this long?
It would not be common - no.
For an ultrasound, would she have to be put completely under anesthesia? My understanding is that sight hounds don't take anesthesia well and given her age I would worry. She hasn't had any procedure requiring anesthesia since she was a puppy so avoiding that sounds like a good idea if possible.
An ultrasound is no sedation / anesthesia and 100% non invasive
That does sound like a better first step than an endoscopy then. thank you.