I would need a bit more information from you to understand the whole picture. For instance, is the Lennar home listed with an MLS, is an offer of compensation sufficient to pay the commission that you have agreed to, and is your agent a member of the same MLS.
I am going to go out on a limb and assume the answer to all three of the above questions is yes.
That said, your agent is likely concerned with a term we refer to as Procurring Cause or, in plain English, what party is responsible for bringing the (Ready, Willing, and Able) buyer to the negotiating table. While different states may interpret cause differently, for the most part this comes down to question of which party maintained an unbroken chain of events in communication with the buyer, which precipitated in the offer to purchase. These events may include: getting the buyer pre-qualified, advising the buyer on the home-buying process (what to expect and when), showing the buyer one or more homes in an area, preparing a market analysis on the subject home (so the buyer is informed on value).
The fact that you walked into an open house (or even made an appointment to view a property) does not in itsself consitiute Procuring Cause. If however the Lennar salesperson was responsible for say getting you into additional showing, getting you approved for a loan and had in other ways followed up with you to keep the Lennar home on your to-buy list, then your agent may have reason to be leary.
To Lexie's spot-on observation below, Lennar is in the business to move houses. Best way to proceed is to have your agent call the Lennar rep to first see if there is a percieved conflict and second work it out if there is (perhaps get the agents' respective Brokers involved). We agents negotiate for a living. This is a situation that can be worked out.
Best of luck to you.