Hi
I had fissures and sometimes after passing stool there would be a streak of blood across it and when I was fully defeated and would clean with tissue paper the was few drops of red blood. Slight burning and then after few minutes like 10-15 I was OK.

A doctor prescribed me gtn cream to be applied every night, and also prescribed duphalac syrup 15-20ml.
He also prescribed antibiotics ciprofloxacin 500mg for 5 days ( 500mg one capsule in morning, another 500mg in evening)
But after this course I noticed that my anal canal has gone so tight that I can no longer pass gas. I feel it has swollen or something.

I suspect it might be caused by gtn? Should I stop it? And just continue duphalac?

The sphincter has tightened up because of the fissure. Unless you have an infection the Cipro will be useless for treating the fissure. If you can get miralax (polyethylene glycol) that's a much better option that the duphalac (lactulose) which creates a lot of gas. Was this a colorectal specialist that you saw?

How you can find out if you have infection of fissure?

You would have symptoms of infection like severe pain, drainage, possibly fever.

it’s not infected, trust me. I thought the same thing when my fissure was bad. Antibiotics are an odd prescription for this but I’m not a doctor