Consider the following situation. Assume that I have two different jobs. Somebody, who knows me and knows my jobs asks me "Where are you?". What can I answer - "I'm at a job" or "I'm at the job"?
The phrase I'm at a job is common for workers who do not work in one specific building or office (for example, plumbers, electricians, painters, or landscapers). They will use this when they are working at some location, doing something like a repair or installation.
If you don't have a job like that, most native speakers would say something more along these lines:
- I'm at my job.
- I'm at work.
- I'm at the office.
- I'm at the hospital. (for a nurse, doctor, or even a cook who works in the hospital cafeteria)
- I'm at the school. (for a cook who works a school cafeteria, or a principal, teacher, etc.)
I think this usually holds whether you happen to hold just one job, or more than one.
You can't say I'm at the job, because a specific single job has not been identified in the discourse, and your interlocutor wouldn't know which of the two jobs he knows about you mean.
And you wouldn't speak of a job unless you were in an occupation where you repeatedly took on different short-term jobs--in which case you would probably say "I'm on a job".
You could say "I'm at one of my jobs", or you could say "I'm at [EMPLOYER]".
But you'd probably say just "I'm at work."