No desire of intercouse. Concern with!

Hi I have concern with regards to my wife's sexual health. She is 30 and feels absolutely no need intimacy. We got married 4.5 years ago, when she was 26 and i was 29. We had 6-7 months of normal sex life, post which we decided to have a child. She got pregnant after 8 months of marriage, and we were blessed with baby boy (with c-sector). After those initial 6-7 months, we hardly had a normal sex life. Our intimacy levels have been negligible since then. We had sex 15-20 times in last 4 years, most of which literally begged by me. I speak to her about this, and she has standard answer, that she doesn't feel like having it / she is in no mood. Worse part, she doesn't recognize this as a problem, and says what's the big deal if we aren't having sex. On the contrary, I feel sexually very active and frustrated at this moment. Its mentally painful to do nothing but self control. It's been 4 years and its hard to believe that i am stuck in a sexless relationship at 33. Please advise.