what do you think it was?

It's possible. My Guardian has reached out and touched my arm, near the shoulder, while I was meditating one time. So yes, it's possible it was your Guardian Demon/ess.Hail Satan.

I was in a restaurant with my BF a few days ago, we were sitting at the bar and we were waiting for our food.
i was just sitting there and suddenly I felt like a hand touched me in my upper back, like literally touched me ' it left so real and i got freaked out and when i looked back there was nobody behind me at all, like 0 people, I could feel like the hand even put some pressure on my back , I've never felt this thing before so I don't think there's something wrong with my body or anything..

Do you think it was a demon who tried to communicate with me maybe my guardian demon?
if was so weird, but I felt good at the same time.

Probably, my guardian makes my muscles twitch or buzz when she's excited. It's different for each person