I am taking phentermine and just started 5HTP - are there any issues taking them together?

Yes, potentially. 5HTP increased serotonin and phentermine can also affect serotonin. This will put you at risk of serotonin syndrome, which is rare but can be serious. You can have rapid heartbeat, diarrhea, vomiting, hallucinations, changes in blood pressure. Possibly life threatening. You should stop one of these.