J.L.
You are going to Puerto Rico one month after giving birth with an infant? Yikes...
Anyway, you have a lot of leeway on what you wear. You can be much more casual than is required. You could even try a mternity dress. I have one that I bought at Motherhood that is a wrap dress of sorts with an empire waist and is black. Look at target also.....You may also get lucky because it is the end of the summer season so there may be a selection on sale. I would go with the size that you wear now pregnant. Don't go too formfitting because you may still look pregnant, but slightly bigger and more comfortable would be the way to go.