If your chihuahua ate more than 200 grams, and it has been with in the past 30 mintues, I would induce vomiting with 3% hydrogen peroxide. Give 1/4 teaspoon every 10 minutes for up to 3 doses. It helps to give a little bread or a snack so there is food in the stomach before you start the induction of the vomiting.
If it has been more than 1/2 an hour, you should take him/her to the vet. The vet is not obligated to report you and I have never known of one that does. The THC in the mariuana can cause neurologic depression and depressed breathing. The signs can last up to 3 days since the THC is stored in the fat.