The Simple Past conjugations of "to be": I was You were He/she/it was We were You (plural) were They were

"Yes, I was." The "worried" is implied in the response, so it's the same as: "Yes, I was (worried)." It's like past perfective in Russian, because you were worried in the past (at that time), and it's a completed action. A past imperfective answer would be like: "I had been worrying," i.e. during that period of time, I was in the process of worrying.

Hi Mannis, The proper response is, "Yes, I was." The reason for this is because the verb "to be" changes in the past tense depending on the pronoun. In the question to which you're providing a response, the pronoun is "you," so the proper verb is "were." But in the response, the pronoun is "I," so the correct form of "to be" is "was." I hope this clarifies things.