My 7 year old DD has suddenly problem with maths, please help

[Praguemummy]

She suddenly started to get very bad at maths. 2 months ago 33 + 50 was no problem for her, even when you just ask her, without writing it on the paper. And now, out of blue she has problem, or mental block as I can see that she is thinking hard but usually comes with the wrong answer. So I try to explain on the simplier example like 13 + 13, she understand we altogether come to result 26 and when I ask her next what 14 +14 she doesnt know again. Doesn't help I have no nerves and start to shout at her, poor little thing I am at the end, as if she wont move on, her school results will go horribly wrong. Please help