I Have Pain In My Left Side Lower (belly) Area.

i have pain in my left side lower (belly) area. I just started on an antiobiotic for URI two days ago but this just started yesterday. I played golf and then went to eat at a mexican restaurant. I had beef tacos and the meat wasn't lean - there was some fat in it. Not sure if this is caused from that or not. I also had three beers on the golf course and then two margaritas at the restaurant. What could this be? It hurts when I touch or push in on my belly and when I walk and move up or down.