I am a Chilean passport holder and entering the US last time I didn't get my passport stamped as usual. Now I don't have proof of entry and don't know until when I can remain in the country.
Not all entries to the US receive a passport stamp, especially if you use the electronic kiosks, and this is absolutely normal and fine.
Try looking up your information on the official I-94 website. That will give you a record of your entry and will tell you how long you are allowed to stay.