Why would my ex girlfriend lie about being pregnant just after we spilt up then tell me she's had an abortion as soon as I have taken her back?

It is likely that she lied about being pregnant and she did this to try and get you back. While this may show her apparent devotion to you, you need to think very hard about where this relationship is going and be VERY careful that you do not make her pregnant if you feel that your life does not include an ongoing relationship with her.