My drivers side brake light went out a while ago and I just replaced it about two weeks ago. Now, I've come to find out that the drivers side brake light is constantly on, and my passenger's side brake light is now burned out. The only brake light that comes on at the right time is the center one in the middle of my back window.

Could this be a fuse problem (this is what my bf thinks), or what else would you suggest? If it is a fuse problem, how difficult is it to deal with myself? I can manage to change lights and stuff myself so I'm not a complete idiot and I can work my way through an owners manual.
OK I need a tiebreaker. What do I do- get new expensive bulbs or take it to the shop...