There is 'onomatopoeia' for words that sounds like what they are describing. But is there such a term for words that feel like what they are describing, words such 'writhing' for one example?
This is an interesting question. The term you are looking for is "ideophone," a word that describes feelings and other things that are not sounds. There is an article about ideophones which is accessible through Google. The article on "onomatopoeia" states that, though it English it means, as you say, words that sound like what they are describing, the original Greek word meant creating new words. Therefore, it seems to me that it shouldn't be incorrect to use "onomatopoeia" for a word that described a feeling, impression, idea, or whatever. After all, all words are sounds. It takes a sort of genius to come up with sounds that convey meanings. Probably all words originated as onomatopoeia, starting in prehistoric times. Anyway, the word you want is "ideophone," and I suggest that you look it up on Google. It seems to me that words like "geek" and "creep" and "nerd" and "blah" and many others are ideophones. Ideophones and onomatopoeia must be closely related, because both necessarily involve sounds. Humans must have a natural tendency to create sounds that express feeliings and impressions.