What Causes Recurrent Nose Bleeds?

I have recently had two nosebleeds that lasted 10-15minutes before stopping. The first occurred two weeks ago, after working all night and staying up the next day helping to take my grandmother s cat to the vet. We had to put it to sleep and then my brother and I buried at my house. The temp that day was probably 80-85f, humidity was not bad, slight breeze and we were in the shade. I took a shower after we were done and when I was bending over drying off it started. Second one was last night after returning home. I had traveled approx 3 hours away and back with friends to watch their son play, he is college football. It was sunny, approx 85-90f, minimal breeze. I used large umbrella to keep the sun off and drank approx: 16oz Dr. Pepper, 16oz Cheerwine, 80-90 oz H2O. When I got home I was bending down to unhook dog leash and it started. The only health problems I have are hypothyroidism, seasonal allergies, occasional migraines. I have been taking for weight loss phentermine and topamax. I check my pulse and pressure regularly and those haven t changed and are wnl. I am thinking it s dehydration but I ve never had this before.