Is my bra causing blocked ducts?

No. Absolutely not but you should seek medical attention to see why this is happening to you.
No. Most likely not. Sometimes certain ducts just block easily or don't release as others. I recommend feeding on the side with the blocked ducts first when the infant is sucking the most vigorously. This helps open the ducts. Other options are heat and ibuprofen. If that doesn't help discuss with your OB or pediatrician. Seek medical care if breast redness or heat or pain.
No. lactation specialists will tell you that tight bras prevent the milk glands to produce milk. so if you want to stop nursing wear tight bras, if you are breast-feeding, avoid tight bras.