Thanks for the query to H.CM. Forum.
As you have discontinue the drugs (pill ) , so bleeding is most common feature .
In your case this type of bleeding is very irregular and that is cause of concern.
Since bleeding is there ,so chances of pregnancy
is nil .
That's why pregnancy test was negative.
Please don't bother , once period will regular , you will get pregnant .
Still in my opinion don't get any test .
At least wait for 2-3 months and thereafter do something.
Good luck for future.