My dogs strange behavior scares me... :(

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Hello I'm Dimitrije and I'm new to this site.
Anyway im here because im concerned about my 3 years old, mix breed, dog's (Bella) strange behavior. In case it's important she is sterilised and vaccined (not right on time but completely)

It started about 1-2 weeks ago, and what she does is that, since im living in a 3 floor house + garage, she either stays in the garage (in her bed) or at the top floor (under my and my brother's desk), it's almost impossible to get her to come down on her own we either have to carry her or to pull her down to eat. And the worst thing is that when she is down in the living room (even when there is no one arround here) she trembles and her tail is under her body like she is scared, her heart beats stronger and faster than usual even though sleeping and playing there was her normal day routine.
She has also became more agresive to other dog's she used to play with (more dominant - even though that is something she always was), yesterday my brother and his girlfriend took her out for a walk and she started attacking every dog she had seen.
Bella has never had any medical or mental problem and since this is new for me I am sincerely really for her (will she stay like this? is she scared of us?).

Last night is when I decided to ask for help here, on internet, just after the terrible experiance of her trembeling and refusing to rest by reacting to every little sound there is, in my bed after I tried to calm her.

Some friends told me that she might be predicting something but i started to think that it has something to do with the fountain and the hose in the garden (kinda connected with the living room), which bella used to visit daily, since a couple of days ago me and my brother wanted to spray her with water since it was really hot that day and she ran under the dining table in fear.

I am so sorry for writing a text this long, but I am really scared for my dog and I am looking for help hoping that something I've mentioned would be of help resolving her problem.

Thank you!