I have a theory on what is going on there......
Becky is making out like she doesn't want kids but maybe she can't have them. I think they want you to think she is pregnant because you saw her with the test, but maybe she has been secretly trying for a while (a la Gavin and Stacey) and has decided she can't, which is why she is being so adamant.
Just a guess, and sure I could be completely wrong, but thought I would share with any Corrie fans out there .