Learn English – Is to talk “under four eyes” a common idiom in English

If you're looking for something humorous, then you'll have to create your own translation, in the spirit of Arthur Waley, the great American translator of poetry from Chinese and Japanese to English. Waley was criticized, unjustly, IMHO, for his brilliant translation of The Tale of Genji by a contemporary translator, Edward Seidensticker, touted as "the best translator of Japanese that has ever lived", but, frankly, I found his "faithful to the original text" translation an utter bore compared with Waley's. Waley was a poet; Seidensticker was a translator. There's a world of difference.

Enough background. I'm suggesting that you create your own phrase. I'm not a poet, but I do have a suggestion. Why not say something on the order of "my heart dropped down my leg and into my shoe, rolled between my toes, then stopped for a full five seconds until I caught my breath again"? You can change things to say what you think will amuse your readers or listeners. Everything else that's been suggested here, including my earlier suggestion of "my heart lept into my mouth", is merely a cliché. Avoid clichés whenever possible.

“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.