Seller's agent says I can't have a buyer's agent! Is it true?

[Ewan Tong]

Hi BPers! I am an investor in Massachusetts. I ran into an open house very accidentally and I was so fond of that house that I signed an offer right on spot! Now, the seller and her agent have accepted my offer. And I realize that I should have a buyer's agent to guide me through the process. I mentioned this to the seller's agent but she says I should have mentioned this before they accepted the offer! They now don't allow me to have a buyer's agent! Is it legal???

Any feedback will be appreciated!!!