gas, bloating lower left side, feeling like i have to go to the bathroom

not sure what is causing this but for the past 2 week i have been having some gas and bloating issues.  I have a gurgling sound on my left side with gas and bloating and pressure.  Its not painful, just uncomfortable. I also get a feeling like i am thirsty but drink a lot of water throughout the day.  I lead a pretty healthy life style, i work out  3 days a week, i try to eat healthy.  I have been doing some research on line and wonder if its IBS or lactose intolerance.  It seems to start an hour or 2 after i eat.  Can you get lactose intollerance suddenly, i am 36 and never had a problem with it before.  I did get wicked diarhea (diarrhea) after eating a bowl of ice cream last week.  I am not an ice cream person usually so it was rare for me to eat it.  I am thinking it might be IBS but not sure, what is a sure sign that you have it.  The only thing that has changed recently is that i swithed multi vitamins to one that has iron in it.  I also take 150mgs of zantac 1 a day, i also just started taking a probiotic again i dont like yogurt.  I tried to make a doctor appt but cant get in to see him till the beginng of may.. anyone else have an idea.  I am going to cut out all dairy and see if that helps.