Constantly getting error 429 "Too Many Requests" despite not sending many requests.

Constantly getting error 429 "Too Many Requests" despite not sending many requests.

I am currently working on developing an integration between our company's app and our zoho books setup using the api.

When testing new functionality, I am very often having the calls fail with Error 429 'You have made too many requests continuously. Please try again after some time.' - Access Denied.

However, when I look at our API Usage details, it shows that I have used 16 API calls today. If I have only made 16 total API calls today, how is it that I have made "too many calls" multiple times? 

This is making development rather difficult, as well as makes me worried about whether the api will be reliable for us to use if we are going to get "Access Denied" regularly despite only making very few calls.