PREGNANT!! trouble with my.... mum??

So I'm 25 and 19+6 weeks pregnant and I'm over the moon and so is my hubby but my mum is my worry at first she was supportive but now she is acting distant and childish with me. I basically used to babysit twice per week but now I'm pregnant I don't babysit much anymore. She had met this guy about a year ago now and it's good to see her happy but if she wants me to babysit and I say no she literally goes in a mood with me and it's really starting to annoy me. 
She said when we found out I was pregnant she would buy us a buggy but every time she argues with me she tells me she's not getting the buggy anymore. I'm fed up with how she's acting with me and I don't know what to do because I've tried to speak with her about it and she denies acting that way. 
  I am at my wits end with my mum and she's beginning to stress me out and that's the last thing I want. But I see myself telling her to f**k off in a nice way and I don't want to do that to my mum but what can I do??!! 
Any help would be greatly appreciated.