Under normal driving conditions, the vehicle in front has the right of way. Merging traffic yields the right of way to traffic already in the lane. Sounds familiar, right? It sounds like the lady behind you saw an opportunity to pass but, may have been caught off guard with the merge or, just got pissed that she didn't get around you. Don't sweat it. If you think you were wrong, let someone cut in front of you later, even out that karma thing.