transwomen are women
Can you explain what definition of 'women' you are using here please?
'cis' isn't a derogatory term at all, it simply refers to the people who identify with the gender they were assigned at birth.
At birth it was noted that I had female genitalia and therefore there was well over 99% chance I was genetically female. The gender which is imposed on female people is a set of roles, assumptions and prejudices which position us at the bottom of the hierarchy, so that male people can exploit our physical, sexual, emotional and reproductive labour.
I don't 'identify' with that and I don't know any other woman who does so of course it's derogatory. There are some fascinating PoMo definitions of gender as some sort of internal 'essence' of 'feeling like' a woman or a man or one of many other inbetween or other things. Those definitions have only been around since the 80's and I don't think they're useful, in fact I think they're regressive.
I'm a teenager and I manage to understand it
OK. As you get older and experience more of how the world treats you, you might find your opinions change. Nobody out there in the real world will care how you 'identify' really, you'll just be 'identified' by others as male or female and treated accordingly. Sorry to burst your bubble.