What is my problem? I can't joke around anymore?

This is tricky to answer. There is a saying......75% of what we worry about never happens. So the short answer is to just joke with them and get over yourself. Maybe you could bring it up at the break table one day that you are afraid to joke around because you might hurt their feelings. I have seen people do that in the work force. Some times people will tell them to bring it on,,.....and other times they will tell them what jokes offend them, so the person knows what areas he can't cross the line. You could also consider that there could be 100 reasons you are feeling bummed from diabetes to high blood pressure to inner ear or sinus issue to a diet with too much junk food, so maybe a physical to rule out something medical and let your doctor know what is going on.