There are many ways to screen you, from detailed and complex to simple or not at all. If she has many good reviews don't worry about it and go have a good time.
Should I be concerned? She is well reviewed but she also knows I'm a newbie with only one available reference. Why would she not check? I'm thinking of canceling but don't want to overreact. Advice?
...Of course I am assuming she wanted personal info since you're a newbie with only one reference. If she is well reviewed as you say then you should be ok.
How well reviewed is she? How many reviews? How many of her reviewers have written lots (10 or more) of reviews?
As xyz said, she may have screened you without you knowing it. Different ladies screen differently. Just a tiny bit of info from you (What did you give her? Phone number? Name? Email?) can sometimes find them a lot of info. On the other hand there are also ladies who screen by gut feeling, and some who don't screen at all (despite all you hear about screening). If this was someone with no reviews herself, then it's a red flag. If it's an established, reviewed provider, then you probably have no worries. In this case the risk is on her end, not yours.
There are many ways to screen you, from detailed and complex to simple or not at all. If she has many good reviews don't worry about it and go have a good time.
Maybe just ask her about her screening methods , before you cancel? Like the others have said, she may have already screened you. 
Some of us just don't put that much weight into references, largely because one lady's experience can (and often does) differ widely from the next lady's. In all likelihood, she verified you through whatever additional info you gave her and didn't see the need to follow up with your reference.
There's also the possibility that she has had issues with contacting that particular lady in the past. For example, if someone uses a local lady as a reference whom I know to be nearly impossible to get in touch with, I won't bother wasting my time contacting her.
Long story short, don't worry about it.
Bills are due, sometimes screening takes a backseat to raising funds
.....I rarely check references. References are terrific, but given the fact that I enjoy another profession, as well as this one, a gentleman's place of employment is more important.
Hugs and Kisses,
Kelly
and that includes women who go on and on about how "exclusive" they are as well. Frankly I don't care either way, but I do find it a bit amusing some times.