Cersei isn't 'changing her mind': she is simply adapting her strategy to achieve her hidden goals.
We know from her conversation with Jaime that she always intended to win the Targaryen's trust in order to betray them later down the line, when they were more vulnerable. Any immediate support would be incredibly suspicious, and as such she needed to present the alliance as being under certain terms, or as a heavy compromise.
Her first strategy is to appear to try and neutralize Jon Snow's intervention in the 'forthcoming war' between Danaerys and the Lannisters by holding him to account on his honor. Cersei has the appearance of gaining an advantage out of the alliance, which neutralizes suspicion.
The events in the South during this episode play quite subtly against the events in the North, where Petyr Baelish later explains the 'rulebook of manipulation' to Sansa; his 'Game' is to ask:
What’s the worst reason they could possibly have for saying what they say, and doing what they do? Then I ask myself: how well does that reason explain what they say and do?
... so, to paraphrase; when judging someone's actions, ask yourself what they have to gain? ... Cersei is playing this very game, by presenting her enemies with an answer to the question.
If Jon Snow truly was, as he had previously proclaimed, not interested in the War for the Throne and focused on 'The Only War' (against the White Walkers), he would have accepted Cersei's terms. Cersei did not know, however, that Jon Snow had fallen in love with Danaerys in the interim, and as such had pledged himself to her. This was not a political move on Jon Snow's behalf, it was an act of Love: which Cersei had no way of foreseeing.
From a political perspective, she will appear in a position of weakness if she makes a demand, sees it rejected, and accepts terms anyway. Cersei has demonstrated herself as anything other than weak-willed, and capitulating at this point would be incredibly out of character; ergo suspicious . Snow's rejection was clearly unanticipated, but she used the opportunity to recoup her strategy.
Tyrion inadvertently provided an opportunity for Cersei's subterfuge; by realising she was pregnant. Cersei then leveraged this perceived vulnerability into an excuse to 'change her mind' about the terms she originally set out.
She would still appear weak/indecisive; but as Tyrion would inevitably report back that he learned of her pregnancy, this decision no longer appears suspicious.