Question for pregnant women!?

Im a 23 year old male my girlfriend is 22 of age we both have a boy which is 3 weeks old.. seems she is not the same no more what can i do she crys for no reason she is mad casue i want to buy the new 3G iphone on july 11 she says she wants to get treated to something even tho im the only 1 payin the bills now.. I cant deal with her stress any more that she dishes out we been together since Aug 5 2007 and she seemed fined then.. what should i do i dont want to stay with someone who gets on my case for no reason she should be happy she just had a gift of a life time a little boy.. we are not married and she also does have a 2 year old son from a prevouis... cant take the pressure anymore