Isn't it possible to add several bounties (to different questions)? I added a bounty to a question yesterday, and for reason, I can't add a bounty to another today...
No, you can only have one active bounty per user as well as only one per question.
Basically any question can only have one active bounty at any one tim and a user can only have one active bounty at any one time.
From the FAQ
There can only be 1 active bounty per question and per user at any given time.
There are some further details on the improvements made to the Bounty system on this blog post.