Your first step is to make a FOIA request. You can find the form at USCIS.gov/forms.
You will be provided a complete copy of your immigration record on CD.
You should also try to get a record of the adoption in the Philippines as well as in the United States.
If you applied for the US passport you should also have the US citizen.
You may have dual citizenship. I would encourage you to hire an attorney in your locality.
Please read also here http://travel.state.gov/travel/cis_pa_tw/cis/cis_1753.html
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IN all likelihood you are a USC, based on your description. Please contact an immigration attorney in person for more information. Also visit www.adoption.state.us.gov. Good site.
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