vent about mother. how do I get her off my back?

Hi ladies!
I'm not sure if this is really a question or more just a vent

I'm 27 weeks pregnant and have 2 children at home with me full time :)
My mum has just been making me feel really down on myself and my life- she's made me feel this way on and off all my life
Anyways she keeps harping at me about our finances. Which in my eyes has absolutely nothing to do with her!
I just don't know what to say to her anymore I've tried and tried to explain to her without going into detail that we can't always afford things that she thinks we should have or be doing etc. We're on 1 wage, it's quite a good wage but obviously we still have 1 wage supporting 4 nearly 5 people. We pay our morgtage and all our bills weekly and have a small amount left after they're taken out for groceries, fuel etc we still occasionally buy 'things'- we don't go without (we don't everything or the best of everything) and we make sure our kids have everything they need. Last night she rang and kept harping on that we need private health insurance but we just dont have the funds left at the end of the week or month regularly to be able to afford this so she started on that we're living beyond our means (we certainly are not) we need to sort our money out better and this and that. She also started about my weight- a very touchy subject for me especially while I'm 6 months pregnant! I am overweight but nothing too major (my Dr's are not worried about and I have made the changes I need to) I'm just so over it! I have been in tears since last night, the things she says are just not needed. Making me feel like we are not doing enough for our family and that I'm not doing what I need to be as a mother :( she makes me feel like a failure!
My sister is a couple if months pregnant with her first baby I'm so happy for her and help her with absolutely anything she wants me to.. she's not working and is quite sick. Mum has been 'babysitting' her (as mum puts it) which is great for my sister. I had a fall last week had to go to hospital and get checked out so I rang mum (partner works out of town) to watch my other 2 kids while I went to hospital she did.. she was nth happy about it made a big deal about it that she had to stop doing what she was to watch my kids and all the rest of it. I was only gone roughly 1 hr as soon as I got home she up and left which was fine I didn't expect her to stay but what made me feel even worse she rang me late that arvo and started having a go at me because I didn't pit the dishes in the dishwasher and I shouldn't have been leaving it for my partner to do... I was extremely sore after falling so I didn't want to push myself too much I had don't the other housework..and partner didn't mind in the least he want me to rest. I don't even know where I'm going with this I think I just had to get it off my chest that she really is making me feel like a piece of crap :( How do I get her to kept some of her opinions to herself and really just to back off-- respectfully?!?!?
Thanks for listening if you got to the end! Sorry my rant was so long!