Mother in law..(says it all really)..?

I just wanted some opinions really. I went to a summer ball on Saturday which was ruined as me and my partner were arguing. When he has had a few to drink he always picks holes in me - I always bite back and we end up arguing. I should really learn to hold my mouth when he does this but I rarely do as its always him that starts it. We do have great times without arguing but had another of these stupid arguments Saturday. It ended with me walking off and heading home (I thought that was the responsible thing to do as I would have been miserable staying when we'd had such a barney, plus it would end the arguing all together!). My other half caught up with me down the road and we argued some more. He stormed off home leaving me there so I sat sobbing on the gr verge until I had composed myself enough to walk home myself. Several minutes later his mother walked down, obviously hearing that we had a barney and I guess followed me down to console me. I was so livid with my man that I was slagging him off saying what a useless so and so he is et etc - you get the gist. Anyways, without warning his (usually understanding) mum said 'excuse me this is my son you are talking about'. I was gob smacked. Considering she is usually the one chewing his ear off about how lazy, blah, blah he is and most of the time she lends an ear when we fall out. You might be thinking yes she should have said this but I am still a bit annoyed about this. I realise this is her son - BUT he is my other half and the father to my 15 month old son. We live together, I am the bread winner in our family as he doesnt work, and we have spent 2 and a half years together. Doesnt this entitle me to a say in MY relationship with him. He is not just her son, he has relationship and financial links to me too. She used to support him financially when he lived with her (before we met) and now its me who see's him every day, brings in the money, washes his clothes, takes the ear bashing when he's in a fowl mood. Why the hell can she speak to me like this? Does anyone agree with me or I am wrong in thinking what I do? It really annoys me as well because she moans about him all the time and when I agree with the the things she is moaning about, we both talk about how we could help change things for the better and work around it. She knows as well as I do that he's a useless git at times, I feel a bit ganged up on now she has opened her interferring trap. I wont be talking to her about anything ever again lets just say that. I look at her in a completely different light now.