Mother In Law Issues

amorebelle
Quick background my husband and I move in with my in laws due to their declining health along with my bother in law. 
We have now completed one year of living together and it is getting hard. 
Tonight my husband and mother in law went at it because of me. 
What happened tonight. 
She came into our room asking for toilet tissue and I said it in the garage I'll go get it. I haven't brought it in sorry and she no BIL can get it when he comes homes and I told her no I'll just bring it in and she tired to stop and I just moved away and went to get it and she was like there's no reason to be hostile and I said no I just want to bring it in. 
(I got mad at my husband because he was like you don't have to be belligerent and I told him seriously you know that's not what I was doing and he said well looked like it. You know how she is)
Now when my husband went to give her medication she goes off on and agrues with her and brings up a past issue that we had resolved. 
When my husband comes back to our room he says that's what I knew was going to happen. 
I'm really at a loss for words. She's so cruel to my husband and I feel horrible but I didn't do anything wrong but I feel I need to say I'm sorry. 
What do I do?