Feel being underestimated

During university time, I used to study together with many men. Females were the minority in that major. However I didn't find it as an obstacle, and I enjoyed studying and working together with them (for one reason also because I found sometimes women are a bit more complicated).

Now I'm in the same situation where I should work mostly with men. But working world is not the same as university world. Totally different. I don't say that I cannot enjoy my work, but for some situations somehow it gives pressure to me. You know, the feeling like being underestimated.
That what often comes up in my mind. Everytime I'm being in the meeting with many people, for example, where I'm the only woman there and see them talking to each other, I sometimes have this in my mind: "what's this woman doing here? she knows nothing."
I believe I still have brain somewhere here in my head, and I don't look like a stupid person (do I?).. but still.. I can feel it.

But maybe that's my problem. To feel too much.
I shouldn't think what they think about me, should I?
Just prove myself to them, that I able to do lots of things also. Even in the area that women are rarely involved.
Anyway, it's easy to speak.