I have been to Caye Caulker a couple of times with my husband. It is great. And I would feel perfectly fine (and safe) travelling alone to Caulker. The people are very friendly. More importantly, everyone seems to look out for each other. Do you have any idea where you might try to stay?
May is a beautiful time to go. We were in San Pedro last May for a week and the weather was gorgeous. Sunny, 80s, rained one time and not for long.
And if you need to relax, Caulker is the best place to go!!!!