No, she hasn't. However, as you stated i suppose it should be assumed unless otherwise mentioned.Women assume you are monogamous by default. If you date at all, you should know that. Now, maybe it was foolish of her to assume you are monogamous, but in general, people who are not monogamous mention that when dating (I'm polyamorous, I'm in an open relationship, I'm into Ethical Non-Monogamy, et cetera).
How about her, did she ever mention monogamy or exclusivity?