Territorial Boston Terrier

I have a 5 1/2 year old Boston, Mona, that is slowly becoming more and more territorial and I'm not sure how to correct the behavior. I got her from a wonderful breeder, fairly mellow parents (for Bostons) and she used to be a complete marshmallow, and got along with every dog she met. When she was 3 I started walking with a friend in the neighborhood who also had a Boston that had some aggressive tendencies. Mona's disposition changed after the first time we decided to play with both dogs together, rather than just walk. We threw a ball into the back yard and instead of both dogs running to the ball, Mona ran for the ball like normal and the other dog, Sarah, headed straight for Mona and attacked her. I should say we were in MY back yard, Mona's home turf (if that matters?). The playing ended right there (didn't want my canine kid playing with a bully). It was shortly after that incident when I noticed Mona being guarded with other dogs, and even a little nippy (only if she was on a leash at first, but now it doesn't matter). The friend and dog moved away, so Mona didn't spend much more time around that aggressive dog. She can no longer "meet" other dogs on a leash. I can't bring her downtown to sit and have coffee because she has to be leashed and she'll take the nose off any dog that looks her way. Help!! She was never like this for all those years. What happened???

Another issue. There is a sidewalk in front of my house and Mona lays at the front door (full view glass in winter, screen in summer) and if certain dogs (not all - seems to be dogs she doesn't "know") walk by and put even one foot on our front lawn, she practically goes through the screen door (and has gone through), barking like a maniac and ruining the screen. She does this whether I'm in the room or not. But she does NOT do this if I'm not home and someone else is at my house with her. Has she decided she needs to protect me? Seems like it. She has become SO territorial and I don't know how to correct it. Any thoughts???