Learn English – How to call 2 friends who are together without mixing it up with “a couple of friends”

“You seem forced” can be used to indicate what you’ve expressed

Your meaning may have been gotten better than you think. “You seem forced” employs a fairly common usage of “forced”, indicating strain even though there is not a physical force. Note that “you look forced” in your question’s title is a little different, having to do with the way that “seem” refers more comprehensively to apparent behavior and “look” refers more to physical appearance.

Another way to say this that would be specific to tone of voice, diction, etc. would be “you sound forced” which could be hedged a little as “you sound a bit forced” and would be helpfully followed by a bit of explanation or a question, such as “is everything OK?”¹ or “do you feel you have to talk to me?”

Expanding on the “look”/“seem” difference in “you don’t look yourself”

With “you don’t look yourself”, it seems like you are saying that something about the other person’s physical appearance is unusual compared with historical data or some assumed expectation (e.g. someone’s face is green or streaked with tears and you assume they are not always that way, even if you don’t know them).

If you were to say “you don’t seem yourself”, your comments would be understood to refer to someone’s overall presentation. This could certainly include physical appearance, but is more often used to refer to someone’s speech, body language, and other aspects of behavior. I believe this phrase would fit the situation you described.

Note for “you don’t seem yourself”: Similar to “you don’t look yourself”, you would be understood to either be making a comparison to ways you’ve seen this person act before or ways you assume they normally act. For example, you think that the person doesn’t normally put so much emphasis on their words or deliver them in a staccato rhythm.

The note from the end of §1 of this answer applies here as well. Since you’re guessing at something about another person, it probably wouldn’t be a bad idea to describe a little more what you mean by what you’re saying. I personally think it would be appropriate to ask a question aimed at learning more about how this person is feeling and/or what they think about the situation.

To say someone is the fastest runner using "number one" you would simply use "number one" as an attributive noun phrase:

She is the number one runner in the world.

This would imply speed, since that is what running races are about, but if you absolutely had to emphasize speed you could say

She is the fastest runner in the world.

where the "number one" would be understood. It would be redundant to say "number one fastest runner" because both mean the same thing in this case. Superlatives are always "number one" and vice versa.

Maui is my number one vacation spot. [meaning favorite]

The U.S. Navy is the number one navy in the world. [meaning biggest, most powerful, etc.]

Heart disease is the number one cause of death in developing countries. [meaning leading, most fatal, etc.]

Some sticklers will insist you hyphenate the term (number-one runner, number-one navy) when using it as an attributive noun phrase, but I don't feel that is strictly necessary. There is little room for ambiguity with the term, and so a hyphen does not lend much support.