Talking about the past in Chinese is simple. Generally, you will use the words.."当时 / 那时候" For example: 我当时在看电视. 当时我在看电视. 那时候我在看电视... More specifically, 我刚才在看电视 <-- you are empathizing "I was watching TV (a very short moment ago)". And... "我在看电视" is a short form. You can also use this phrase to describe "I am watching TV now".

当时我正在看电视。

我当时在看电视。

我刚才在看电视

我在看电视...