I don't think you're assuming too much. I actually had the situation a couple of weeks ago where I received a "hope to see you at recruitment weekend, and by the way, here's some fellowship details" informal email from the chair of a department but no official admission email or letter, which made me paranoid that they had mixed me up with someone else. As it turns out, the hard copy of the letter and two separate official admissions emails (one general admission and one with my fellowship package breakdown) all managed to be lost in transit. So yeah, it definitely doesn't hurt to ask.
And if you're invited to recruitment weekend, doesn't that mean that they're trying to, well, recruit you? Thus implying that not only did you get in, but you should be expecting a fairly competitive financial offer?
