
It wasn't that you were writing for another company which caused her reaction.
It was more that she was probably surprised to learn that you were already working for them under one of their umbrella companies.
These umbrella companies are set up to con students into signing up with them for their papers while covering up the fact that the student may or may not have already had a bad experience with the company previously.
You can most definitely write for multiple companies, that is the point of being a freelance writer. In fact, there was a point in time when I was still actively writing for these companies that I was signed up to work independently for 6 of them at a time.
Back then, multiple companies run by the same people wasn't the norm yet.
So when I applied, I got in and did the job.
It was only later on that the companies started selling their shares to other companies in an effort to either consolidate their business and eliminate competition or, to start the scam operations coming out of various European countries.