No, Trina, you are not obligate to use her as your buyers agent - unless you signed a buyer agent agreement with her. A disclaimer - I am in CA. So, IL might have different rules, but I have not seen that on this board.
The only one I know might do that is New Home Developments. Quite a few new home developers have the rules of requiring an agent to sign you in when you first view the home; and if you don't have an agent who brings you to view the new homes; then even if you have a Realtor, your realtor can not be involved if he did not sign you in. That's the one thing that might be different. .
As far as resale is concerned, I don't think you are obligate to use the agent who showed you the house because the listing agent would not come and show it to you.
In this case, i will be especially careful about who I pick as a Realtor - somebody who will protect YOUR interest instead of theirs.