My tegus turned out to be really robust. One time they had hexamita which wasn’t pretty. They became less active and ate hardly or nothing at all. The result would have been death due to hunger.
But a short visit to the vet and an excrement-test gave a quick result.
To prevent something like this you should have the excrements tested once a year. That costs around 20 Euros but you know if your animal is sick and if it is you’ll know what the problem is.  

With (only germany,maybe you could tell me an us link) that’s really easy. Sending the excrements… pay… get the results. So you won’t directly have to go to the vet.