Your concern is reasonable, and I've thought about it myself. The Wikipedia article linked downthread by int_19h notes: "Up to one percent of all those detained in immigration detention centers are nationals of the United States according to research by Jacqueline Stevens, a professor of political science at Northwestern University." There are specific cases mentioned in the article, and the case of Mark Daniel Lyttle was pretty alarming. It was written up in: https://www.newyorker.com/magazine/2013/04/29/the-deportatio...
On the other hand, you can't be detained without probable cause, and race/ethnicity alone isn't enough. For instance, the Supreme Court ruled in United States v. Brignoni-Ponce (https://case.law/caselaw/?reporter=us&volume=422&case=0873-0...): "In this case the officers relied on a single factor to justify stopping respondent’s car: the apparent Mexican ancestry of the occupants. We cannot conclude that this furnished reasonable grounds to believe that the three occupants were aliens. At best the officers had only a fleeting glimpse of the persons in the moving car, illuminated by headlights. Even if they saw enough to think that the occupants were of Mexican descent, this factor alone would justify neither a reasonable belief that they were aliens, nor a reasonable belief that the car concealed other aliens who were illegally in the country. Large numbers of native-born and naturalized citizens have the physical characteristics identified with Mexican ancestry, and even in the border area a relatively small proportion of them are aliens. The likelihood that any given person of Mexican ancestry is an alien is high enough to make Mexican appearance a relevant factor, but standing alone it does not justify stopping all Mexican-Americans to ask if they are aliens."
I second the recommendation to get a RealID. You're going to need one eventually for domestic flights, among other things. When I got mine at the DMV renewing my driver's license, they asked for a birth certificate, social security card, driver's license, and proof of (local) residency (e.g. utility bill). So why not get one and carry that as additional proof?
On the other hand, you can't be detained without probable cause, and race/ethnicity alone isn't enough. For instance, the Supreme Court ruled in United States v. Brignoni-Ponce (https://case.law/caselaw/?reporter=us&volume=422&case=0873-0...): "In this case the officers relied on a single factor to justify stopping respondent’s car: the apparent Mexican ancestry of the occupants. We cannot conclude that this furnished reasonable grounds to believe that the three occupants were aliens. At best the officers had only a fleeting glimpse of the persons in the moving car, illuminated by headlights. Even if they saw enough to think that the occupants were of Mexican descent, this factor alone would justify neither a reasonable belief that they were aliens, nor a reasonable belief that the car concealed other aliens who were illegally in the country. Large numbers of native-born and naturalized citizens have the physical characteristics identified with Mexican ancestry, and even in the border area a relatively small proportion of them are aliens. The likelihood that any given person of Mexican ancestry is an alien is high enough to make Mexican appearance a relevant factor, but standing alone it does not justify stopping all Mexican-Americans to ask if they are aliens."
I second the recommendation to get a RealID. You're going to need one eventually for domestic flights, among other things. When I got mine at the DMV renewing my driver's license, they asked for a birth certificate, social security card, driver's license, and proof of (local) residency (e.g. utility bill). So why not get one and carry that as additional proof?