1) make sure you wear a condom every time you have sex for the best chance of avoiding her "accidentally" getting pregnant.
2) Sit her down and let her know, in no uncertain terms, that you do not want children, at all, and that is not likely to change, and explain why that is the case. DO NOT say "maybe I'll change my mind later" because that would just give her false hope. You don't sound like a fence-sitter to me, you sound like a solid No.
3) Ask her why she seems to he changing her mind about this issue you've previously agreed on. Does she just feel differently? Is she being pressured by friends/family? Has she always actually felt this way but hoped you would change your mind?
4) Figure out if you can come to a complete agreement as to whether you will or will not have a child. There is no compromise here - you either have a child or you do not. And "just having one" IS NOT a compromise because that's still having a child.
5) If you cannot come to an agreement that makes you both happy and is a permanent decision, you have to break up.
And #1, make sure there are no needle holes in the condom package. It happened before to people.