Wait another three weeks and you will know for sure.
Was she in season when the dog visited? Were they left alone together?
Has she put on weight and are her nipples harder and pinker?
These are signs she could be pregnant, but none of the behaviour you have described is normal for a pregnant bitch.
I would suggest you get her checked by a vet because she could have something wrong with her, and she is biting and acting strange because she is either in pain or is in distress. It could be a phantom pregnancy, or even pyometra.
I cannot understand this reluctant to take a dog to a vet - although I don't take a dog without good reason, if there are any health worries I take them straight away.