Any antibiotic is likely to cause diarrhoea with watery stools due to upset of the normal GIT bacterial flora. But the pain during BM is not a part of this. This is unusual and needs to be checked up. Also the blood in the stools.
You have to approach your family physician and get yourself checked up. Any abnormality in BM which presents like this shoud be checked up to rue out more serious causes. The pain could also be due to causes like anal fissure, a small tear in the anal mucosa. Other causes which may be possible are Crohn's disease, Ulcerative colitis, tumors, etc. But you must get checked up to rule these out.
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