It sounds like they already did make you go downstairs and take a breath test.
That being said, if you are in a private place where you have a reasonable expection of privacy, the police cannot enter without sufficient probable cause or an exception to the probable cause requirement. Whether they had that in your case depends on the specific facts of the case. However, the police can ask you to do many things that you have the right to refuse, such as perform field sobriety evaluations, consent to a search of your person or property and give a statement incriminating yourself. In most circumstances the police do not have to tell you what Constitutional rights you have and that you can refuse their requests. You should hire a lawyer to help you with your case and since you are under 21 you should not be drinking alcohol.
Be aware that, handled improperly, an underage drinking violation can have all sorts of bad "collateral consequences" including losing your license. Be sure to consult with a lawyer.
You can contact me at my office at 770-744-4211 or by email through this website. The questioner and any reader do not have an attorney-client relationship formed by our communications on this website. Advice given by me on this website is general advice based on partial information. You should not rely on any advice given without first hiring a lawyer in the area where the case is pending, and providing that lawyer with full information.
An owner of a residence is entitled to allow their own child, even underage to drink alcohol, but the same rule does not apply to the friends of the underage drinker. As stated above, you have the right to refuse to come down, but it sounds like you voluntarily came down and submitted to the exam. In doing so, you give up your rights.
If you have any further questions about this or any other topic, please feel free to call me at 404-923-0446. All initial phone calls are free. By answering this question, there is no established attorney/client priviledge with Rohan Law, PC. Therefore you cannot rely solely on this information to form the basis of a legal opinion regarding your rights and responsibilities. If you believe that your case requires additional attention, you should contract an attorney to represent you directly.