Trans means that you have a gender identity that does not align with your biological sex. 'Cis' means that you have a gender identity that does align with your biological sex. So what does that mean? 'Gender identity' itself is about feelings, characteristics, and personal expression.
I don't have a gender identity. When you call me 'cis' you're assuming that I have a feminine gender identity to match my female body, when in fact I have experienced femininity as oppression since I was a very small girl. I don't identify as loving high heels and shopping, as being emotional or submissive, or with most of the other characteristics that have been culturally marked as feminine. The same characteristics that many transwomen say are what makes them women.*
As a woman who rejects sex role stereotypes (which is what we used to call this 'gender' business) and who is a feminist, yes, calling me 'cis' offends me.
*It's invariably some combination of liking feminine clothes, hair, and makeup, liking a sexually submissive role (see 'sissies' - or don't, unless you have a strong stomach), or else some kind of inchoate feeling they claim is womanhood, to which I say firstly that we're individuals, not a hive mind sharing the same womanly feelings, and secondly, no man knows how it feels to be any woman, any more than I know how it feels to be a man, a horse, or a tree. He might know how it feels to be uncomfortable with male stereotypes or with his male body. That's not the same as being female.