I smoked bowl of weed 1.5 months ago and have a hair test coming up, was clean for months before that

How about after that?

If that is really all you did, then you are probably ok with this test.

A hair drug test is meant to show consistent use over time. A 1.5 inch sample of hair, cut at the scalp line, will go back 90 days. One bowl of weed most likely will not make you positive on a hair drug test, unless the bowl was the size of a cereal bowl.