YOur doctor's response was interesting. Oxytocin is the hormone that causes your milk to let down. But it is a hormone that is released in many situation. Notably, it is the hormone that is released during orgasm. Has your doctor advised you not to have sex or orgasm? (I would be surprised if he had!) If you miscarried and had had sex would you always blame sex?
There is absolutely no doubt that you have breastfed long and well - and when you remember back you will remember you fed her for about 2 years (you have!) You are well within your rights to wean. The hormones, as the dr says, of your new pregnancy will limit the milk supply. And oxytocin released during breastfeeding will not cause you to go into labour until your body is ready to go into labour anyway. The choice to wean or not is yours - but breastfeeding does not cause miscarriage or premature labour