It's difficult to say (I'm a [retired] Medical Librarian. So I don't dispense medical advice. I may suggest things to you that might be plausible or tell you something from my own experience, but I'm a breast cancer patient, not a Melanoma patient. I researched Zelobraf and of course, skin rashes from nearly any drug could be a sign of an allergic reaction, so the best thing for you to do is to get in touch with the doc who prescribed it asap or the on-call doc at your hospital. If you find your lips swelling or any difficulty breathing, of course, dial 911.
There's no way without examining you that anyone on the forum could really conclusively rule oout an allergic reaction, so it's important to contact your doc or the one who's covering on the derm or oncology services.