What Is The Cause And Treatment For IBS?

Hi, I am a 33 year old male living in US, Around 2013 September I was given some strong antibiotics for sinusitis since than I started having unformed/short stools with around 3 times a day went to GP and he suggested it may be IBS and referred to GI specialist, went to GI specialist and he told to take Miralax and also suggested Colonoscopy. I did not feel any improvement and I was really reluctant to have colonoscopy done. I called doctor and asked if he can do a FIT test. result of FIT test came negative. Then doctor said take Miralax and OTC Probiotic for another two months and see if any improvement. In the mean while I visited India (My home country) and consulted GP who got altrasound done of my stomach which showed everything alright including GI wall thickness, she suggested it may by IBS and referred to GI specialist there. GI specialist told it should be IBS and gave some medications including some probiotic for 15 days. Unformed stools went away within that time. I returned to US but since than I have been noticing a mix of short and flat stools. I don t have any other symptoms. Eating some foods like cheese causes shorter stools and some constipation. I take a lot of water now but I am worried if it could be something serious. Again I am reluctant to have Colonoscopy. I would appreciate your advice. Thanks.