How to be a warm,good demeanored person.

I first realized that I might give off the wrong vibe a few months ago at work. I worked in a clothing store, and a costumer told another employee that I was rude to her, and had a bad demeanor. I was really busy that day, and didnt even remember talking to her, but I still feel really bad that I had upset someone.

Then a short time after that me and my boyfriend broke up. For some reason I could not get along with his friends. Were all super nice people, but for some reason they thought I didnt like them (Im assuming its my demeanor again).

And now in my pre-nursing class, we practice nursing skills on each other, and the partners I work with look at me funny, because of the way I world/say things while I pretend they are my patient.

I really don’t understand why Im coming across a "cold" person, I truly feel like I care about people no matter what. Has anyone had an experience like this? I just want to be able to come across how I really feel. Thanks :)