I got arested for battary because my ex called police and said i hit her i was at work at the time . ?

I hadn't answered to her for a few days I have txt msgs of her saying she was going to my work to talk to me after she went I told her to just leave me alone and I didn't want nothing to do with here anymore 30 min later cops show up saying I hit her I have witness that seen the whole thing why did I still get arrested now I have a dv charge ..how can I prove I didn't hit her