I'm surprised at comments that say that middle class is completely divorced from income. I always assumed they were quite closely related, with a couple of exceptions.
Upper class is clearly limited to those who are aristocratic or related to peers by birth or marriage. Totally separate to income, I agree.
However, the distinction between middle class and working class I always took as being related to education or earnings and not at all related to birth. This is what encourages aspiration in the UK, that everyone is able to achieve more than they are born to.
In which case, a single parent earning almost 100k would in my mind be middle class no matter what their family or job.
I was solidly working class when I was born but I worked damn hard to where I am which I think most would consider middle class and I am not ashamed of that.
The couple of exceptions I mentioned would be those working in creative industries such as artists and actors, where income is inherently unpredictable, and in those cases I would probably emphasise the relevance of education in determining whether an impoverised artist was working or middle class, but I do see that as a flaw in my logic.
Anyhow, OP is middle class in my view, however she earns her income.