This is a tough one. Your mum is taking out her frustration and feelings of helplessness out on you in a way which isn't fair at all, but it's kind of normal - who else does she have to vent to about her concerns and fears? Same with you. Maybe you should try to get your mum to come to IHD - she might feel support from the caregivers on here.
I think we all understand how it can be on those off days. I understand not wanting to share a lot of it with your mum... you don't want to add to her burden (but since you live with her she'll know anyway.. she's not your mum for nothing you know!). She may even resent in a way if you don't share because she can tell you're not having a good time yet not sharing.. so she may feel pushed away or something.
It's a tough road to tread absolutely.
I think, if it was me, I would sit down with mum and just say how you're feeling. be honest. Get it off your chest. I mean you're worrying about it and it's affecting you and stressing you - none of that is good for you(or mum) so just try to find a good time to sit down over a cuppa and be open and honest with her about where you're at and how you feel. She may appreciate a little more where you are coming from as much as having things aired.
Good luck!