A -te form verb followed by から and another phrase has two possible meanings. If what follows is an action, it means that the first phrase happens, and then the second does. In other words, the second phrase happens after the first. If what follows is a state or condition, it means that the condition has been true since the first phrase happened.
'学校が始まってから忙しいなぁ。'I sure have been been busy ever since school started.'