Please note english is not my native language, it's my first time posting

Some background I'm 17 and coming from a poor, bad neighbourhood in Eastern Europe, semi-jokingly called "The District of Flying Knives" in other parts of the city. (Surprisingly) I've never been bullied, robbed, involved in serious crimes or seen anyone get badly harmed. Seldom did anyone try bothering me. I used to be shy but also friendly to people, non-confrontational.

However, on many occasions over the last few years, new friends told me that–before they got to know me better–they thought I'd looked intimidating. Things similar to (badly translated): "You are very kind and considerate, but I thought you were about to beat somebody up at first" Very odd, also a bit funny considering I was always skinny, wearing normal clothes. Neither tall nor able to keep long eye contact with strangers.

I read two or three "casual" psychology books "for dummies" one and a half years ago. Started paying attention to straight posture, confident handshake, looking people in the eye, smiling and laughing more, asking questions/showing interest in conversations etc. Learning to use my voice by recording commentaries online also played a big part.

I got into one of the better highschools in my city a year ago. A similar situation to the ones before occured. People seemed more anxious and even afraid talking to me than I was to them, and I was still, mildly put, shy and very insecure deep inside (frankly, I still am). Yet, after some time, I've become good friends with nearly everyone I talked with.

I found TRP this year and have read R. Greene's "48 Laws of Power". It is still difficult for me to conciously use the knowledge in practice. I started working out more seriously in May and it has helped me more than anything else. I feel a lot less stressed, having fun instead of being nervous. "That's it" I thought, "I was just anxious and stressed–that's why people thought I looked aggresive!".

Certain that the problem was solved, I started working as a tourist guide in my city's Institute of Culture this summer. The first few trips went very well, everyone was more than happy. Every few months the Institute offers free PR-oriented 16-hrs courses for it's employees. The first I got invited to was about "Self-presentation and public speeches". After all that happened, I already felt confident about talking to an audience but I decided it would be a good idea to check it out.

One of our exercises was to prepare an individual 5-min speech, then present it in front of everyone. The other participants would grade it and write their comments down on anonymous notes for the presenting person to read and learn what he or she did right/wrong. Out of 18 participants other than me, there was one man. Aged 30, married, two kids, did not appear confident. The other 17 were women. Aged 20-45, besides a fresh architecture university student they were all much older than me so I felt zero sexual pressure (like I probably would if they were all pretty highschool girls). My speech went very smoothly, it was the most positively-graded one actually. However, in a few negative comments, I found familiar phrases again: "intimidating posture" or "looked like was about to attack somebody" etc.

I don't get it. May it be caused by the enviroment I was brought up in? What can I do about my posture to look more "friendly" to strangers? Are there some awfully obvious "aggressive body language signs" I miss? Or is it even a thing that should bother me?