Can I take a diuretic if im on phentermine? I feel extremely bloated in my abdomen. Thank you

GI or water. In my experience, constipation and diarrhea are more common side effects from phentermine than water retention. Instead of adding another drug, my first advice would be to INCREASE your water intake to see if that helps. But if your bloat is ONLY in your abdomen (no swollen extremities), I think the culprit is more likely to be constipation/gas.
No. Being bloated in the abdomen is not a valid reason for using a diuretic. It's not likely to help and there are risks of dehydrating all your organs (particularly your kidneys) because of your perceived excess fluid in the abdomen, which, if present, would be an indication of a very serious disease.