How do I deal with a husband who lies to me about anything to do with an old girlfriend of ours?

He is NOT having an affair with her, for sure. He always had a soft spot for her, as did I till a year ago when she and I entered into a business venture together as equal partners. She behaved unethically with me so I closed the business coz I did not want to ruin our personal relations, though I felt disillusioned. When I told my husband about her behavior, he neither sympathized with me nor had the courage to tell me that perhaps I was wrong or had misunderstood her. Instead he chose to sympathize with her. I felt hurt, but let it go. However, after that incident, my husband started lying to me about anything to do with her. If he got a phone call or text message from her, he'd say it was from someone else. He'd invite her to dinner or to a restaurant and not tell me, so that I'd find out only when she landed up. If I traveled on work, he'd go out with her, not tell me, and lie to me that he stayed at home while I was away. He denies lying to me at all. What do I do?