Responding to ad hoc requests

This is my first post here so I hope this is the right place.

A couple of weeks ago, a client sent me a text message stating she had something she wanted me to look at that might interest me. I promised to visit their office but was unable to due to other work commitments. At the end of the week, she reminded me about the issue and I visited them the following week. During the visit, she asked me to put in a proposal to do some work for them. I worked flat out the rest of the week and weekend to get the work done by Monday. On Wednesday, she asked me to submit a second proposal for a related task. I suggested I could complete it by the next Monday but she was adamant I could get it done sooner. As I had other work I had put aside while I did their work,I pulled an all-nighter so I could get the work out of the way and continue with the work I had begun.

At the weekend, the lady asked by text if I could complete another assignment on her behalf (i.e. do work she was expected to do for her employer). I left it until Monday to respond. She offered a small amount of money and said the work needed to be done as soon as possible. I offered to submit it by the end of the next week and she insisted she needed it done soon. I explained that I had other assignments I was working on and would get back to her at the end of the week. She said I should stop my other assignments and complete her task first. I offered to find someone else to do the work and she said she didn't know I was busy with other work and would "do it (herself)". I tried to explain that I needed flexibility to respond to ad hoc assignments and she said she didn't understand my explanation but thanked me for it.

I've been freelancing for less than three months now and wondering how I could've handled this differently.