Our moods and how we react with different people do vary.
So if you are with a sad person, that too will make you feel sad. You are in fact picking up the vibes from that sad person.
Being with a happy crowd will make you feel happy too.
You are fine and quite normal.
Your profile says that you are a female. A few days before a menstrual cycle you may find that you will get mood swings, and that is also normal and due to hormonal changes. If it was very severer then you would need help with this.
Bipolar is usually onset later in life but it can be early onset. Mine became onset around 15 and that was my first manic spell. This website has a good mood tracker where you can post daily and keep track of your mood. It will not diagnose you but it will give you insight on how your mood changes and the symptoms you may feel.
Other than that I think you are fine and I wouldn't over-think it.
It's probably just empathy. People have these things called "mirror neurons" in their brains. They help people copy and learn behaviors. For example, when we see someone who's clearly very depressed, our brain will create the same neural pattern. This is a large contributor to feelings of empathy between people, even when you don't consciously know that the other person is sad. This also influences people in positive ways, for example people will often gain energy when around people who have a lot of energy.
As for when you're in public, that's just how people are. People have highly fluctuating moods.
Bipolar disorder is distinct in two ways.
1. Moods are extreme and severe, including depression and mania/hypomania. They include disturbances in sleep, appetite, and other basic functioning.
2. These extreme moods are sustained for at least 1 week for mania/hypomania, and at least 2 weeks for depression.
If you think that you fit these basic criteria, please feel free to contact me, I'd love to help in any way I can.