Did the forms have the initials of INM? If so, then you signed something from the Instituto Nacional de Migracion who controls immigration to/from Mexico. Did they take additional details like passport information down?

I think if it was INM that did this to you and that they took down passport information I would be concerned, as that information would make its way to into a database.

But it seems to me based on current information, they merely detained you and forced you to sign something without asking for additional information or searched you.
If this was secondary screening then at the minimum, they would have searched your belongings.

If you feel you have time, you can visit the Mexican consulate and see what was noted about you or attempt to get a visa or take a trip to SAN and cross the border into Tijuana on foot and see what happens (not the best way to find out). Or if you don't want to find out, and you want visit the DR again, skip any layovers which happen to go through Mexico.