Why is my blood pressure low after I had my teeth extracted? Is it because of the anesthesia?

Possibly. If you had oral, im or IV sedation, you might have some residual effect from the medication which could lower your blood pressure for up to 24 hours. It would be best for you to consult with your surgeon.
Depends. ..On how soon after. In some cases, relief at completion of the procedure coupled with orthostatic hypo-tension (dizziness when you suddenly get up) can cause a significant dip in blood pressure immediately post-op. If it has been awhile since the procedure and your blood pressure is outside the normal range, get it checked out.
Possibly. You have hypertension and you may have taken the medication prescribed for it that day. While not eating or drinking before surgery you may have developed a relative hypovolemia, or low blood volume. Anesthesia can make these symptoms worse temporarily. This can be easily treated with hydrating. I hope that this helps.