Need advice, was written up by my t. Director Friday, and main boss will be back tomorrow - can I be fired?

The bathroom in our building is WAAAAY far from my office. So, I had to pee really bad, and was on a conference call. So, I just peed into this empty water bottle while I was in my office on the call. A coworker came walking by, and just laughed (he's a friend), but then when I went to pour out the pee into the sink, he started freaking out and TOLD ON ME to the t. Director, who then went ballistic saying that was disgusting because people fix their lunch at that sink, etc. Even though I was careful to pour it right into the drain. He wrote me up, and now my main boss returns later this morning. Can I be fired for this?