I dove into a muddy waterfall in the dominican republic and now, two days later, i have a headache and rhinitis - what's going on?

Entirely new: There are several potential triggers for rhinitis that include infection and physical environmental changes like weather and temperature amongst others. Travel is a common trigger in and of itself. A consideration is whether this is a new symptom or exacerbation of an underlying problem. If entirely new, then infection would be high on the list of possibilities (needs to be evaluated).