Was the offer negotiated verbally or it was presented to you in person/via email?
If the offer a verbaly low offer, f.e., and your agent just ran it by you as an option for a while (not even thinking you'd consider this offer) - and then, when everything firmed up, she presented the offer officially and disclosed her status with the buyer - this may not be a problem...
If this was not the case, then the proper disclosure was not done timely, and this needs to be discussed with your agent and her broker.
Re. the buyer - why would the property be shown to the buyer, if you are in contract?
Unless this is a short sale and you want receiving back up contracts, you should not be showing the house any longer.
Every realtor, who is both a listing and a selling agent, has to treat both clients fairly and ethically.
In most cases, it will work fine, as long as you are happy with the terms of your contract.
Hope this helps,
Beachfront Realty, Inc.