ESTA is clearly not a work visa - it only allows for business or tourism activities. You are not allowed to regularly work in the US if you are currently there on this visa - in my opinion (certainly I am not a legal expert on this).
Ask yourself if either of the following is true:
- Do you have an employment contract in the states with the US company you mentioned?
- Do you receive a salary (taxable income) from this US company for work you are doing (inside the US)?
You are allowed to visit the company for business purposes, but if you started working before you receive the appropriate visa, that is generally illegal.
A very good answer to this possible related question can also be found over here: https://travel.stackexchange.com/questions/9245/uk-citizen-us-company-living-in-uk-can-i-use-an-esta
In essence, you can not work for a company, unless they already employed you in another country and you are just there for a business trip. If you have no prior employment relationship (in another country) with this firm - things really become pretty clear - in a way that you are not allowed to work in the US right now!