What causes farts that smell really bad and a lot worse than normal?

Giardia?: May things can upset the balance in your GI tract causing gas, bloating, and even changes in stool. Infection (recent antibiotics?), giardia (travel hx?), change in diet (might you be milk or wheat sensitive?), etc. You can try "pro-biotics" (lactobaccillus), or charcoal tablets, & focusing on a healthy, balanced diet (note some foods cause more gas, see Dr. Google). If persists, see your doctor.