Poor Ruby Gloom. You came on here as a newbie asking an innocent question and got flame grilled.
To answer the question, from my experience, hormones do change a dogs disposition. A bitch in season, or a pregnant bitch, or a nursing bitch with a litter will all act differently at different stages, so , yes, the behaviour change you are experiencing COULD be pregnancy related. But it could also be resource guarding as stated in an earlier post. Either way you need to put a lid on it and stop the behaviour, no matter how "sweet" you may think it is.
Labs are usually very friendly to all people and if well socialised, good with all dogs as well. Like all dogs though, if they have not met and mixed with other people and dogs, they will not let others near you, because they have a fear of the "unknown".
Get your self and your dog enrolled into a club with experienced trainers - my preference is a club that only trains using positive reward methods. You will find a link to 2 clubs in Perth here ADAA: Clubs List
On the matter of breeding RG, I am not going to rehash what everyone else has already flamed you with except to ask, have you really taken in what has been said here? You started by telling us you had bought the bitch to breed with, but in your last post you tell us that you will spey her when she is mature. Did I read that right? If I did , then congratulations for accepting this forums advice and thank you for listening. I would however suggest that at 18 months, your Lab is mature and you should therefore book her in for the op now, today.
Border River Pet Resort