I'm having horrible thought and feelings towards a severely autistic child I know

I am not being rude but you need some psychiatric help. Your post natal depression already explains a lot.

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I'm wondering if anyone can advise me on why I may be having these thoughts.
I have recently met a woman who has a daughter with severe autism. I find her too familiar and after only knowing Her a few days she was arriving at my house several days a week and just walking in with her daughter and staying for hours. I've managed to stop that now by ignore the door when she knocks but one day a week I see her at a toddler group we both go to. I find her annoying and suffocating at times but I wish her no ill and I know she just had trouble understanding social boundaries. However, my feelings towards her 3 year old autistic daughter are scaring me.
For some reason I find her irritating, unappealing to be around and worst of all,I constantly have thoughts of harming her. I know i would never actually harm her but the thoughts are there. I don't want to feel this way and cant understand it.

I was diagnosed with post natal depression which may explain my feelings but in getting much better now and never felt this way about anyone before.

I have OCD but very mild and never affected my life before. Maybe the depression has made it come back(it had gone away).

Any ideas? I'm a 24 year old female with a 13 month old baby

Good luck getting it on the National Health, if you're lucky they'll just prescribe some anti-depressants.